how do I delete photos

Here’s an intro to the Photos app on Mac: Use Photos on your Mac - Apple Support

See the section Delete photos and videos from your library

If you’re using iCloud Photos, deleting the photo in one place deletes it everywhere.

Do you want to delete all photos or just some? If you want to delete all photos, delete the Photos Library. By default it is in your Pictures folder. You may want to keep a copy of the library on an external volume, so you can restore the photos, if you want them back.

If you just want to delete selected photos, it might be a bit confusing after the Catalina upgrade, because the command "Delete Photo" does no longer appear in the "Image" menu or the contextual menu, if you are viewing an album, only "Remove from Album".

But you can bring "Delete Photo" back to the "Image" menu and the contextual menu by holding down the command key ⌘ while viewing the menu. The ⌘ key will change the menus and bring the "Delete ... Photos" command back to the "Image" menu. That is why the keyboard shortcut ⌘⌫ will delete any selected photo anywhere, even while viewing an album.

See: How to delete photos while viewing an Album in Photos 5.0 on Catalina - Apple Community