Mr. FB. Bonus for having a drain hose on the sump. Your speculation as to the "wine stopper" is close but no ceegar.
The end with the annular rings (the piece minus the end cap is (last picture), termed a hose barb-in this case, a male hose barb) should remain in the rubber hose, at all times and tightened by means of attached hose clamp. It is meant to stay in place. With two wrenches, you can unscrew the cap. One wrench on the hose barb and one on the cap. Remember "Righty-tighty...Lefty-loosey.
Once the cap is off you can attach an oil sucking device. Potentially with a matching female hose barb to another rubber hose to a sucker to a pail...
I use an almost identical set up temporarily connected to a Reverso oil pump. One of these years I want to make a manifold to eliminate fiddling around with oily hoses as you're about to do.
I'm sure you get the idea.