How do you decide on your opening?

in lichess you cant really study up on your opponent, each one is new when you go to for quick pairing. For those who pick from the quick pairing section, how do you decide on the opening you will use... In my case I randomly start my games with e4 or d4 as white and e6 as black... is this meant to be a random decision or is there another way of arriving at that first move?

I think you have to gauge your performance in certain lines. I don't play as if I am playing different people. I play with moves already assessed. At the end of a game, go over it with an engine. For each questionable move where you don't fully understand a position, decide 2 or 3 possible candidate moves. There doesn't have to be a best. As you play stronger players they will gravitate toward certain moves over others. You can then know when players don't do this, they might not be as strong. You can then start an attack earlier.

#1
"All main openings are sound" - Kasparov
So just pick a defence for black against 1 e4, then a defence for black against 1 d4 and an opening for white and then stick to it. Play it all the time against all opponents and in all time controls so as to accumulate experience.
So one game against an opponent in one time control helps you against another opponent in another time control.
It helps to select openings with some synergy between them.
E.g. Sicilian 1 e4 c5, and Benoni 1 d4 Nf6 2 c4 c5 as black, and English 1 c4 as white,
Caro-Kann 1 e4 c6 and Slav 1 d4 d5 2 c4 c6 as black and London 1 d4, 2 Bf4 as white,
King's Indian Defence 1 d4 Nf6 2 d4 g6 3 Nc3 Bg7 and Pirc 1 e4 d6 as black and King's Indian Attack 1 Nf3 2 g3 3 Bg2 as white, French 1 e4 e6 2 d4 d5 3 Nc3 Bb4 and Nimzo-Indian 1 d4 Nf6 2 c4 e6 3 Nc3 Bb4 as black and Rossolimo 1 e4 c5 2 Nf3 Nc6 3 Bb5 as white,
Latvian 1 e4 e5 2 Nf3 f5 and Dutch 1 d4 f5 as black and Bird 1 f4 as white,
Scandinavian 1 e4 d5 and Grünfeld 1 d4 Nf6 2 c4 g6 3 Nc3 d5 as black and Center Game 1 e4 e5 2 d4 as white...

The point of an opening is to get to a middlegame position that you understand. The positions you're aiming for should influence the choice of opening that you pursue. Although middlegame positions have been the topic of entire books, you can break down the structures into a few main ones. Fundamentally, the pawns will drive the flavour and the ideas behind the middlegame and you should play the opening with an eye on what type of pawn center you wish to achieve, bearing in mind any concrete tactical considerations as your opponent may not necessarily oblige you.

- Open centers (no pawns occupy the central files): Develop fast and effectively, don't waste time on meaningless moves.
- Closed centers (each side has two pawns in the center, interlocked and unable to move): Slower with maneuvering, setting up your pieces and looking to break on the flank with a pawn
- Fixed centers (each side has one central pawn that is blocked/restrained): The fight revolves around controlling key outpost squares and occupying them with your pieces
- Mobile centers (one side has a mobile central majority): The side with the mobile majority will try to control squares in front and use pieces to support a push to create a passed pawn, while the other side will try to restrain such movement and trade down to a winning endgame
- Dynamic centers (unresolved but will transform into one of the other types): Try to steer the position into on that is favourable for you. If you have a spatial advantage with the "big center", you want to cramp your opponent's play, prevent them from undermining your center, and avoid exchanges, converting any advantages into an attack

Different openings will tend towards different structures, but both you and your opponent have some flexibility. Focus on structure over memorising lines, bearing in mind tactical and positional considerations to steer the game in a direction that you're comfortable with.

I play only one opening as white and about same as black; this way I do not have to choose :).

Sicilian as black, four knights as white.

by fair dice roll

I just use the master database for the openings I usually get, for about the first ten moves and then go with whatever feels the most comfortable, also if I see magnus has played it too I'm more confident about it.

#8
Magnus plays some weird stuff...

Scotch opening for white french defence for black