What is this red and neon yellow thing? Kinda looks like a bracelet?

https://sh.itjust.works/post/56986342

Looks like a pretty shitty prusik loop. Usually used in climbing or other ropes applications. A good prusik would be of a better rope quality and the knots would be more robust but same principle.

more like a fisherman’s knot. When you pull on it, they will hold tighter

en.wikipedia.org/wiki/Fisherman's_knot

Fisherman's knot - Wikipedia

Yes, except that it appears to be a double fisherman’s. It’s hard to tell, though, tbh.

fisherman’s knot always looks like that till you tug on the line/rope to lock it tight.

up there in the loop they just used it keep the size adjustable