regarding AI body modular construction: I think it's not unreasonable to conclude, based on what we've seen, that the vast majority of AI bodies aren't really post-market modular. i mean, they *can* be modded, and it may not even be that hard, relatively, for a lot of them. If you have the parts with the proprietary connectors or ID chips to greenflag interface with the rest of the body. or have the know-how to fabricate them.
like, manufacturing realities will probably mean that a variety of limbs can be connected to a chassis, for example. among other things, that lets you more easily market a stable of variants. You may even offer a 'build a body' service where a more well-to-do buyer can mix-and-match variant options a la dress-up browser games, then put them together and ship out after the check clears.
(also maybe there's a relatively flimsy connection-tab or weld or something added as part of the finishing that voids a warranty if it's broken, but *has* to be broken to swap out the limb, to discourage non-first party repair/maintenance)
on top of all that, we've seen scant evidence that most body models are capable of basic self-service/hot-swapping. which to be fair, most AIs it's come up with (which to also be fair isn't many) seem to share human squeamishness regarding uh... their inner workings, we'll say. Which, fair. but also would probably imply there's not a lot of pressure on manufacturer's to offer the capability on that front, either.
personally i suspect they're on the level of laptops, at least where laptops were a decade ago when i last paid attention: moddable, with some work and perhaps some risk, but not accessibly modular at all to the common user.