APKMirror is run by the team behind Android Police. They host signed, unmodified APKs of original system apps.
For 99% of users, the DocumentsUI that shipped with your Android 11 device is exactly the one you need—bugs and all. Documentsui Apk Android 11
If you have a rooted Android 11 device and want to replace the broken OEM file manager permanently: APKMirror is run by the team behind Android Police