Cybertronians don't 'interface' to make little cybertronians, and while frame transplants and 'cold-forging' are a thing, both still require a compatible Spark as a base, and both of the latter (but especially frame transplants) have a lot of issues, from full frame rejection to varying levels of frame dysphoria due to not truly matching their Sparks. (Note: Technically 'cold constructed' bodies can be painstakingly matched and designed to match their Spark, which eliminates these problems – the issues are related to the industrial production of product lines, rather than frames matched to individuals.)
Before you can do any of that though, you need a Spark. There are two three main ways of getting this, all of which were used to varying degrees during the Age of Expansion:
1: Titan Sparks put out so much energy that they can support the growth and deployment of Newsparks from onboard HotSpots. This however is not something a Titan can just decide to do – it requires cooperation between Titan and CitySpeaker, and much of the Titan's energy has to go into this process. This often formed the initial stages of various colonies, where a Titan would be sent out as a Colony ship, land, and slowly build up a population in line with how quickly the colony could be built to support them. There are limits to the number of Sparks this method can produce however, and it is quite Energon intensive, leading to these colonies being dependent on external supplies to keep them going. Those produced in this way also tend to take elements from their Titan, so a Beastformer Titan will result in a primarily Beastformer colony, etc.
2: The second option is of course The Well of The Allspark and its connected Hotspots and Colonial Wells. This has always been the way the majority of Cybertronians are created, as well as the seed populations of Mechafauna and Mechaflora that maintained Cybertron's systems.
For much of Cybertron's history, this has also been how most Transformers would have their equivalent of families, either through a Sparkling literally stumbling across them or going on a quest to one of Cybertron's hotspots. Towards the end of the Golden Age the Well of All Sparks was covered over to allow the Sparks that came forth to be harvested immediately, placed in protoform bodies and taught to transform so that they could be assigned to a Guild and function.
This has widely been regarded as a mistake, since while it was indeed 'more efficient' it also effectively prevented middle and lower class Cybertronians from mentoring others and the poor state of many of the bots that came out of this was a major contributor to the unrest that characterized the lead up to the Great War.
3: The third option is the most similar to biological reproduction, though most humans would consider it more akin to a medical or engineering procedure than anything else. Two or more 'bots can have small cuttings from their Sparks removed – the more bots that donate, the less each one needs to contribute which reduces the recovery time – these are then placed together in an Energon-rich nutrient solution where they will merge together and form into a new Spark that can then be allowed to develop or placed in a new chassis as usual. In many cases donors will also donate code snippets which result in the Sparkling being more prone to take on traits of one or more of the donors. This method is technically independent of Primus, however the current state of the war makes it infeasible – not only is it not an environment many wish to bring new Sparklings into, but the process requires a lot of Energon both for the Newspark and to help the donors recover, as well as weakening them for some time. Even before Cybertron went dark, this was mostly used by Colonies without Hotspots, wealthy established code-dynasties on Cybertron such as the House of Ambus, or bots with some wealth that was extremely determined to have a Sparkling that specifically took after them.
If you don't have at least one of the above? You don't get Sparks, period.
So, lets assume you do have a source of Sparks in the form of one or both of the above. What now?
Well, now we have to talk about how those Sparks go from motes of pure energy to living Cybertronians.
In this, there are three main ways it can go:
1: The 'Natural' way, with no additional support until they have created a Protoform.
The Spark settles somewhere rich in Energon and with CyberMatter nearby and forms an initial Protoform around itself. This Protofrom can continue to grow and change as they develop, either with or without the help of a Mentor. This is quite time consuming and relies on both of the above being had in abundance, so tended to only really happen on Cybertron or other highly developed Colonies.
2: Custom-Made Chassis
The Spark is captured and a body is custom built for it to be implanted into, which is then modified, expanded or even replaced as the Sparkling grows. This has most of the benefits of the first option while also being faster and safer for the Sparkling. It is also the closest they get to the organic method of child rearing, in that this involves one or more Cybertronians closely monitoring the Sparkling as they are raised in order to make sure everything develops properly.
3: Constructed Cold/Cold Forging
Introduced after the Well of Allspark's production slowed during the Age of Rust, this effectively skipped all the painstaking customisation and mentoring of the first two and simply implanted sparks into production-line chassis with existing programming. This could be said to be the most 'efficient' method, as these Constructed Cold Cybertronians walk out of the factory knowing everything they need to know to function in society or as a soldier, but effectively skipping the Sparkling stage and being in a body that often just does not feel quite right can lead to many mental issues and sometimes severe Frame Dysphoria. Ironically, given it was the Council and upper caste's decision to make them in the first place, these bots were often discriminated against and thought little better than machines thanks to their initially stilted and simple speech.
From the first two options, you get Sparklings:
Sparklings are surprisingly capable for such young creatures, having a variety of inbuilt programming that allows them to find energy and other resources, recognise danger and – if needed- defend themselves. However they are still young and weak, and there is only so much their programming can help with, especially if resources are more scarce.
This is where Mentors come in, effectively parental figures who, either alone or working together, help protect, provide for and educate these Sparklings in everything that isn't part of this basic survival programming.
In some cases a Sparkling will form with no mentors present and simply develop as it will until it encounters someone.
Most Cold Forged bots are just bots like anyone else, but they most well known examples are the Vehicons.
Vehicons are the result of Shockwave's experiments regarding both third options – originally with the goal of mass producing copies of Megatron's best warriors, he instead found that if you did not care if the donor survived (or, in fact, it was preferable they didn't), even a single Spark could be shattered into many shards – usually between sixty to a hundred twenty depending how strong it is. Likewise, being more economical with the nutrient solution allows these Spark shards to stabilize but are stunted, being unable to power more powerful weapons and chassis and with less well developed free will, at least initially – they can and do still slowly grow, but as disposable grunts most do not survive long enough to do so.
Combining these shattered sparks with cheap and mass-produced chassis results in a Vehicon, one of the main reasons for the Decepticon's numbers advantage later in the war, as they could turn captured Autobots into yet more soldiers.
