For probably a year and change, our Model 3 was smelling not-quite-right when you first got into it. It got worse with all of the non-use through much of last summer. I try not to think about how maybe it was from driving through sewage early last year (some dude had an RV spilling his sewer load and I couldn't do anything to avoid driving over it). Then I happened to be reading through the owner's manual to the new Model Y... yes, I actually read it this time and learned all kinds of stuff... and one of the few things it mentions for scheduled maintenance is replacing the cabin air filter every two years. We're almost three years in, so...
There are a ton of videos about how to do it, and it's kind of a pain in the ass because you have to remove two panels and reach a screw in an inconvenient place, but I wouldn't characterize it as hard. All of the videos seem to recommend the same HEPA filters, and they all suggest using a can of foamy cleaner on the coils too, since you're already up in there. I'm happy to report that the improvement was dramatic and immediate, to the point that the interior of the car has no smell at all anymore. The old filters were in pretty bad shape.
This got me to thinking, gosh, every time that we've oscillated between AC and heat in our house this winter, the AC smells funny from the upstairs unit. I'm not saying it has irritated my allergies, which have been terrible, but I'm all for ruling out potential causes. I assumed that conceptually there probably wasn't any real difference in how you clean the coils in a home unit, so I ordered a couple more cans and cleaned those too. We ran the heat for an hour or so this weekend, and there was no stink when I switched to the AC.
And this is all cool (see what I did there?) because it's a bunch of simple tasks that I'm sure would cost hundreds of dollars to have someone else do it, so go me. Hopefully we'll be breathing easier.