I'll attempt a general answer to the question about finding replacement parts from different models of the same Shimano component, but it actually fails in this specific case.
Go to si.shimano.com, which is Shimano's center for manuals and technical documents. You can search for the component series, e.g. M8100, and scroll through the components, or there are a number of other search features.
I believe the parts diagrams are marked with the logo
DM stands for dealer manual, and
UM for user manual. The latter two manuals can be accessed by browsing the regular Shimano site, but the parts diagrams can't be accessed that way. I'm not certain what Shimano's term for this diagram is, and I'm going to call it parts diagram until I learn otherwise.
The diagram for the M8100 brake levers is here. It calls the lever the LH or RH "lever member unit", and it provides part codes for either one in case you want to try to get one through an LBS. This diagram also gives interchangeability for all parts with the M9100 and M9120 units. Unfortunately, it didn't list backwards compatibility. I've seen Shimano list backwards compatibility on other components, e.g. various R8000 parts diagrams have (I recall) listed compatibility with 6800 components (these are both Ultegra road components, different generations, roughly equivalent to Shimano XT). In any case, A means identical parts, B means interchangeable but the appearance will differ.
The M8000 parts diagram, i.e. the one for the previous XT generation, is similarly unhelpful for the OP's case. It lists interchangeability with parts from M9020, M9000, and M785 components.
While this doesn't work for the OP, I'd suggest it as a first step for similar cases. Another thought is that people may sell damaged components for parts on eBay, and it may be possible to determine if the individual part of the component is working. Alternatively, single parts (e.g. a left-hand brake lever or a right-hand one) might get listed because the seller damaged the other side, and decided to just upgrade their bike.