Make browsing the internet possible only on Chrome, Safari or Edge (with no modifications or extensions). No competition allowed in browsers.
Make browsing the internet possible only on macOS, Windows, Android or iOS (no custom Android distributions, definitely no LineageOS or GrapheneOS or whatever). No competition allowed in Operating Systems, especially no open source operating systems.
Make crawling the internet possible only to Google. No private crawling and no competing search engines.
Let me know if I've missed anything...
Make browsing the internet possible only on SoCs allowed by Apple, Microsoft, Google. No competition allowed in SoC. [0]
Make browsing the internet possible only on form factors approved by Apple, Microsoft, Google. So no calculator with a web browser [1]. No competition allowed in form factor.
Make browsing the internet possible only on UX approved by Apple, Microsoft, Google. So backtracking 10 years ago, when Android made documents-oriented web browser (= each tab appears just like a standalone app in recent apps), that would have been abuse of that position. No competition allowed in UX. [2]
PS: I come from Android OS world, all those examples already apply to Google/Android.
[0] Well this one will depend on whether their Web Environment Integrity implementation will enforce full secure boot approved by them. Considering how it went for Android, I'd say it will, but can't say for sure.
[1] Yes you can find calculators running Android (but can't run Google/Android so no Chrome). Amongst a lot of other weird Android devices. You can find walking robots, toothbrushes, urinals running Android.
[2] You'll probably find a better example. Arguably it's the same as "competition allowed in browsers", but that was an OS-wide change, but saying it's "OS" IMO largely reduces it.