Endless ads to clean up my iPhone 15

Have you saved some related website (web app) on your home screen? If so, remove that, and see if the notifications stop.

If these are notifications—and not email spam, or SMS messages, or something else—then:

… Change notification settings on iPhone - Apple Support

… Use notifications on your iPhone or iPad - Apple Support

If you’ve installed an add-on VPN client app, or an add-on “security” app, or other such, remove that, and test again.

Have you recently installed a new App? Possibly a new game? If so, this is most likely what is generating the ads.