Put my PC back as it was

Uninstalling AngAinOne does not put Windows settings back to their original state: what you had applied stays in place. Uninstalling does, on the other hand, keep every backup — nothing is lost, and reinstalling the app finds them again automatically.

Where the backups are

C:\ProgramData\AngAinOne — optim-snapshots (the state from before each tweak), powerplans (power plans exported as .pow), dlss-backups (the original DLSS DLLs of your games), driver-backup (exported GPU driver packages).
%LocalAppData%\AngATV\AngAinOne — per_game_drs and game_registry_snapshots (original 3D settings and registry keys, per game), nvidia_defaults_backup.json (DRS settings from before any change), game-config-originals.json (original in-game configuration), dlss_swaps.json, amd_registry_origin.json and intel_registry_origin.json.
%AppData%\AngAinOne — Snapshots (debloat, tweak and preset snapshots).
Registry — HKLM\SOFTWARE\AngAinOne and HKCU\Software\AngAinOne: StartupBackup, UsbBackup, DnsBackup, DnsApplied, OptimMarkers.

How to roll back

No button undoes everything: several pages restore item by item, and the power plans page deletes instead of restoring (see below). Reinstall AngAinOne, then, depending on what you had applied:

Tweaks — the "Restore catalog tweaks" button. It only covers the tweaks shown on that page.
Appearance — three "Restore Windows defaults" buttons, one per section: File Explorer, Folders in This PC, Taskbar and Start. Each one only touches its own section, and nothing is written before "Apply". The "Interface & behavior" section has no button: turn its switches back one by one.
USB — no button restores USB. "Turn USB sleep back on" forces sleep on every controller: that is not a rollback. MSI mode and interrupt priority are undone by no button: set them back on each controller’s card (the "MSI mode (latency)" and "Turn off USB sleep" switches — unchecked = sleep allowed — and the "Interrupt priority" list). The values from before are noted in the registry, under HKLM\SOFTWARE\AngAinOne\UsbBackup: the app never reads them back, use them as a reference.
Startup, Network, DLSS — restoring is done item by item on the page (program, network interface, game): there is no global button.
GPU drivers — the driver packages exported before a purge are files, in C:\ProgramData\AngAinOne\driver-backup: reinstall them from that folder, the app does not do it for you.

Power plans: the "Restore" button does not restore yours

On the Power page, "Restore" puts the default Windows plans back — and deletes your custom plans and the manufacturer’s along the way. They are exported as .pow to C:\ProgramData\AngAinOne\powerplans before the operation, but they are not reimported automatically: to get one back, use powercfg /import "path to the .pow" in an administrator terminal.

If you do not want to restore anything, the locations above can be deleted by hand — rolling back will then no longer be possible.