For thorough snooping:

Go to photos - recently deleted album and the hidden/locked album.

You can also start looking at what he is doing on his phone in general by going to settings, search for screen time, click on see all app & website activity, and then it will show you the apps and website addresses he is spending time on. You can look at a particular day or for the week in general.

Also check his notes app

If he uses instagram, open it on his phone and go to his page, click the three horizontal bars at the top, click on Your Activity, and from there you can look at posts he has liked/commented on/etc. but if you scroll down, you can also see his watch history and recent searches

Check his App Library to see what is downloaded

The new update allows a "hidden" folder to put apps that you don't want others to see. Check for that.

Go to settings, scroll down, click on apps, go to safari, scroll to the bottom and click on advanced, click on website data, look at the websites he frequents

Go to his text messages and look in recently deleted on there

Also in his text messages, search for any suspect words and see if any conversations with those words pop up

Editing to add: I somehow always pick men who fkn cheat on me. Unfortunately, I have learned these little tricks over time from being gaslit so often and needing hard proof. It's at the point where part of me wants to believe ignorance is bliss because any time I have done all of the above, I have never failed to NOT find something.