Backups & restore points · Data Backup basics
Which files does Data Backup skip by default?
Short answer: New backup sets skip Trash folders, Caches folders, .tmp files, .DS_Store files, node_modules folders and Git's internal object store — things that are rebuilt automatically and not worth versioning.
Every new backup set skips a few things that are almost never worth keeping versions of: anything in a Trash folder, Caches folders, files ending in .tmp, Finder's .DS_Store files, node_modules folders, and the internal object store inside Git repositories.
These are rebuilt automatically by macOS, apps or developer tools, so versioning them would only make backups bigger and slower without protecting anything you'd want back.
If you're a developer, note the Git point: your working files are backed up, but the repository's internal history store isn't. Keep your code pushed to its remote as well — see Git repositories and Data Backup.