There is a clamp on bridge mount that fits over the muzzle end of the pistol, tightens against the picatinny rails and holds a small scope or red dot above the slide. It must be removed to field strip with re-zeroing each time. That is it.
The thin, zinc slide does not allow threading, dovetails, etc.
I recently filled the rear of the anti glare band on top of the slide where a red dot would sit. Then glued down a custom made thin aluminum base that was drilled and threaded for a very light 4 moa Shield red dot. It works fine and the pistol has no problem cycling it. There is a recent thread on it somewhere around here. 1917
I have used the Walther bridge mount on a P22 for years. It works pretty well but is somewhat of a PITA to mount and dismount. Its a 3 piece affair and I was always dubious but it held up fine and would generally hold zero when mounting/dismounting.
When I got my 9mm PPQ, I got a UM Tactical bridge mount. It is one piece, and has a snap lock to mount. Might be a little heavier duty than the Walther. Same bridge works on the PPQ 9mm, PPQ .22, and the P22. They also offer a holster that grips the mount so it can be used with any gun.
Walther bridge mount on a P22. I think these are around $35 and work well except for needing to be removed for field stripping. Once installed and zeroed mine held up fine. It is made of tough polymer just like the grip. 1917