What's /system/vendor/lib/liboemcrypto.so? and where does it come from? I'm on lineage os and I have an app that crashes because this file doesn't exist on my sytem. When I do an internet search for the file, I see that there are plenty of people getting things working by removing it, or renaming it to liboemcrypto.so.bak or something, but no one seems to be missing this file. Can I get it from anywhere?
That's (part of the) implementation of Android's "modular DRM", which came into being in Android 4.3 (API level 18) -- see MediaDrm. Essentially it's the component that allows media player apps, under certain conditions, to play back DRM-protected contents.

If you are running on Android 4.2 or lower, however, you're probably out of luck. To confirm, you can install an app like DRM Info, which will tell you whether any DRM library is installed on your system. If not, am afraid there's nothing you can do on this specific device.