Elo loss if a 900 player defeats a player with1,300+ elo.

Let’s say a 900 player defeats a 2000 player. How much elo will the 900 elo player get? How much will the 2000 player lose?

Try it yourself

Rating loss/gain always depends on both players RD.( rating deviation, higher RD = more elo gain and loss in a single game)

My RD is around 50, and if I lose to 900, (2200 vs 900), I guess I will lose around -20 to -24 points.

Elo gain to 900 player depends on his RD.

This is basically correct, just some slight specifications: the higher YOUR RD (which stands for rating deviation, basically a measure of how much on average you will deviate from your rating; this is once again oversimplified of course) the more you gain/lose, however the higher your opponents RD the less your rating will change. However your RD makes a much bigger difference than your opponents.

Additional info to above post. As my RD is 50, and if I lose/win vs exactly same rating, I lose/gain

6 rating.

Lichess does not use ELO

To elaborate on @BT_555 's comment, Elo is a specific rating system which is named after Arpad Elo. Rather, Lichess uses the Glicko-2 rating system. You can find lots of info online that explains it better than I could. :)

ELO can reference A system of rating systems Or it can reference a particular rating system. Lichess does not use the Original ELO rating system so in that sense it does not use ELO but it uses Glicko-2 which is a variant upon the Original ELO rating system. And ELO can refer to the systems that improved upon the original ELO system. So in that sense, Lichess does use ELO.

That is not true. ELO references the Electric Light Orchestra. I am willing to accept an argument that it references their first album. But it definitely does not reference a rating system.