Mac says the disk is almost full but i can't see what's using it

Restart before you go deleting things. Swap files and some of the cache only clear on a reboot, and it's not unusual to get several GB back just from that. If you've ever installed Xcode or Docker, those two can quietly sit on tens of GB as well.

When 'System Data' is the huge bar, it's often old iPhone backups and Time Machine snapshots. Things to check: - System Settings > General > Storage, click the (i) next to each category, it lists the big files - old iPhone/iPad backups: plug in or open the device in Finder > Manage Backups, delete the old ones - Time Machine local snapshots: in Terminal, tmutil listlocalsnapshots / shows them. They clear themselves once the Mac backs up to the drive - Messages attachments, Downloads, and the Trash, which doesn't empty itself If you still can't find it, GrandPerspective or OmniDiskSweeper draw a map of every folder by size, and the big one jumps out.