Home → Apple/Mac Help → Airplay → Sequoia 15.2 Screen Mirroring
Check to see if you are "Currently Mirroring" or "Currently Extending"
To change between Mirroring and Extended -- Click the change button
If you want to switch to extended display -- Click the "Extended Display" icon and then the blue Extend Display button.
If you want to switch to mirrored display -- Click the "Entire Screen" icon and then the blue Start Mirroring button.
Note -- When you disconnect from the apple tv and reconnect, you will stay in the mode you were last using when you disconnected.