Really interesting question!
I have an unusual EV made by a relatively small company of which only a handful got to private customers, so if I want to fix something, I have to reverse-engineer it first. Most of the time, I will find out that the components used in my vehicle were also used in other cars.
Regarding the difference between EVs and ICEVs, only the powertrain components are relevant and between those, some are more exchangeable and some are less so.
As with ICEVs, most manufacturers have "platforms" that are shared between multiple makes/models. Having shared components with other vehicles of the same platform is the rule rather than the exception.
In the cars I have seen, the whole battery often only fits that specific model, sometimes also for other cars within the same platform. The modules that make up the battery are often exchangeable with other cars made by the same company/group. The cells that make up the modules are almost always generic, but very hard to replace. The battery management system is usually specific to the battery.
I don't know about the current state, but for early EVs the motor and inverter (which converts battery DC to AC for the motor) were often made by external suppliers. Especially EV variants of otherwise ICE-based vehicles like the Fiat e500, VW Golf/Jetta, and some french cars all use the same motor and inverter made by Bosch. If an inverter is connected to a different type of motor, it needs to be tuned for it which is not trivial.
Onboard Chargers (OBCs), that convert AC line voltage from AC chargers to battery voltage are often quite generic and developed and manufactured by suppliers. They are almost always interchangeable within the same platform, but I haven't yet seen completely unrelated OEMs use the same OBC. The same applies to fast charging communications equipment, which is often integrated into the OBC.
DC/DC converters (the alternator equivalent) are rarely separate components anymore and often integrated into either the OBC or the inverter.
Voltage-wise, all these components are often surprisingly flexible and can be used with much lower voltages than their maximum rated voltage.
Other components like contactors and connectors are very generic and I haven't yet seen one that only one OEM would use. There are likely exceptions to this. Often, the base components like the OBC or the inverter are almost identical, only using other (also generic) connectors.
While technically all these components could be replaced in the "old school" style, almost all of them require either coding the components to the specific vehicle, or flashing an OEM-specific firmware. While the former is only doable with OEM-specific software (that is far too expensive for both indiviuals and most independent workshops), I haven't yet seen any example of the latter, at least not for swapping components between unrelated platforms.
As of now, there are almost no "official" aftermarket replacements for these major components. I don't know of any major supplier that will directly sell parts in small quantities and OEMs likely won't sell you as an individual replacement parts either. For DIY repairs, finding used parts from wrecked cars and coding them with cracked software or having it done in an authorized workshop (if even possible) often seems to be the only option so far. Also, everyone will discourage you from working on your EV for "electrical safety" reasons (actually, it's more profitable if they do the work). Working on an EV is quite safe, if done right (which is not hard).
Most of these limitations do not only apply to EVs, but to almost all modern cars. Often, the necessary work of reverse-engineering and cracking software has already been done for ICEVs for tuning purposes.
