The 10mm allen key cap that covers the 8mm allen bolt must stay in place, that is essentially the crank extractor.

Reinstall and tighten the 10mm cap, now loosen the 8mm bolt. It will loosen and then feel tight again, the bolt is pushing against that 10mm cap to push the crank off.

That will remove the one crank arm, you will likely have to give the protruding axle a tap or two to drive it and the attached drive side crank out of the bottom bracket.

Now you will need an external bottom bracket tool to check/tighten or remove the BB cups that are tightened into the bottom bracket eccentric insert.

Lastly you will want to remove/tighten clean/grease the eccentric BB itself, it will have an allen bolt holding it in place. That particular one looks like it tightens from the side with another allen bolt, probably in one of those little holes. Even with the bolt loose it might seem stuck as it's a giant wedge, again slight tapping might be required.

Edit: fixed some words. Ignore the bit about the wedge style eccentric BB, this is a split shell clamping style. Generally the least creaky of the eccentric BB's, assuming those bolts are tight I would tend to think it was the crank, BB cups or chainring as you suspected.