How do I switch back to my internal drive to boot from?

I have a late 2015 imac running Big Sur. A year or so ago I asked about ways to improve performance, and ended up following the suggestions to buy a solid state drive to boot from and to essentially replace the internal fusion drive.

I purchased the recommended hardware and ultimately never saw much of a difference. At this point I'd like to switch back to using the internal fusion drive so I can use the expensive solid state external drive for other purposes.

The way it's set up currently is that everything is on the external drive, and the internal fusion drive is just used for time machine backups.

What is the process I would follow to essentially clone the external SSD and put it back on the internal fusion drive, so that I could remove the external SSD? I'm not sure how to go about this. Thank you for your help.