Why do I always get or lose 8-10 elo, but my opponents are 20, 50, 100...?
That mostly means that they are new or long inactive players.
There are two components of the rating change. The first looks at the difference between the two competitors ratings. The second looks at statistical measures of how accurate the rating is considered. When someone is new or hasn't played much in a while, that component can lead to larger changes.
When you're new to the website, or haven't been on in forever, it tries to figure out your skill level. As such, your elo goes up and down like a roller coaster, before finally stabilizing if you play consistently.
If you actually want to get more elo faster, you should play opponents who have a higher rating than you, within reason. In a game with a major elo unbalance, the lower-rated player wins a lot more elo if they win, and vice-versa.
For example, if a 1000 plays against a 1500 and wins, they'd most likely get about 12-14 points from them. But if the 1500 wins, they'd probably get only 3-4 points.
Why do I always get or lose 8-10 elo, but my opponents are 20, 50, 100...?