Can someone explain some of the things the guy is doing in this OP-1 video?

Like flipping between different layers, muting all but one layer? Stuff like that?

Yup you got it right.

Start to 30s he’s using the number buttons to play/mute tracks.
If you let the track play with no buttons pressed it will play all tracks.
If you press a track button it will play that track only. If you press two of the buttons then it will only play those two channels.

At 31s he hits the shift and arrow right key to jump to the next sequence loop within the tape (watch the tape advance a second following this.)

When he hits buttons 7 or 8 it applies user-created effects. Buttons 4/5/6 are also effects.
4 = tape stop
5 = plays tape in reverse
6 = stutter type effect

1.55 he’s holding down a (pitchwheelbent) note.
(Pitchwheel is shift and the arrow keys)
then tweaking the green knob to control a parameter in the current synth.
He continues to play with the pitchwheel effect.

2.14 he slams the stop button to give tape-stop effect then immediately hits play again

2.26 he hits the shift and left arrow keys to jump tape to a previous sequence - you’ll see this skip back on the screen a second later.

2.51 he’s adding effects to the overall output - adding a delay then tweaking it to increase the effect then at 3.00 or so switching efffect off.
3.17 switching it back on againthen turning effect down.

Hope that’s enough of a help

Are sequence loops the green ins and outs you can create? How do you make multiple loops?

yeah that’s correct. the IN (button 1) is the start of a loop and the OUT (button 2) is the end and as long as you have button 3 clicked (loop symbol) that loop will continue to play.

Say your loop is a bar, to jump to the next bar use the shift and right arrow whilst playing the first.
When it gets to the end of the first loop the markers and loop will then jump to the next bar to the right.

You make multiple loops along the timeline (tape) and jump back and forth between them.

There are loads of handy tutorials on YouTube made by some of the guys who hang about on here - they are a million times more helpful than someone trying to explain the OP-1 using text …

and of course TE’s own videos

That was one of the vids that confirmed it for me.