openings

I would say it is better to stick with one opening if you want to play for a win but it can get boring after a while. If you want to play for fun you can play as many as you want but probably you will have worse performance that way. The middle ground is to learn more variations of the same opening, this way you can score well and still bring some new ideas to your games.

Do both. Learn one opening really well, but also experiment with different openings to familiarize yourself with a lot of different pawn structures.

When you are starting out, try different openings and then play the ones you do well with that are also good openings. If you reach a slump, this could be a good time to try a different opening but I would recommend keeping your repertoire somewhat consistent with what you are comfortable playing. For example, the Carlsbad structure is very common in the Queen's Gambit so choose variations that lead to that middlegame and learn how to play that pawn structure so you have a plan. Oftentimes, people lose in the middlegame because they don't know what to do, and having a familiar pawn structure will help you a lot

Both are fine. The objectively better approach is probably too stick to one opening and really understand your way through the opening. But learning different openings and different structures and ideas are good too.