Update:
I reinserted the oil plug.
I then removed the old oil filter and a little old (black) oil started coming out.
I then started pouring the new oil into the engine to see if it would come out of the oil filter hole. No new oil came out.
I installed the new oil filter and poured all of the new oil in (7 liters).
I removed the oil plug again and I got around 2 liters of old oil to come out.
None of the new oil seemed to ever come out, even with the oil plug off.
I would assume that this means that something is plugging up the system?
Also, there was a plug, a spring, and another metal cylinder piece. I don't know if I reinstalled them correctly. I made a sketch, seen in the link below...
Here's a photo and a sketch:
https://imgur.com/a/FWXC04p
Thank you!