Traditionally you use a fuck ton of water in a giant pot but, really, that’s mainly so you can boil it harder without it boiling over so easily. The actual pasta comes out basically the same regardless of the amount of water.

What isn’t the same, though, is the water. And many pasta dishes call for adding some of the starchy pasta water as a thickener/emulsifier for sauces, so actually if you’re doing that you’re better off using less water as it’ll be more concentrated. And obviously less water boils faster.

For this reason many well regarded chefs have taken to cooking pasta in a wide flat pan with just enough water to cover it, at least for small quantities.

Serious eats has a good article: https://www.seriouseats.com/how-to-cook-pasta-salt-water-boiling-tips-the-food-lab

Depends on your vessel. Honestly, it doesn't matter at all. It's not like rice where the water becomes a significant portion of the finished pasta. Just have enough water to cover the pasta without having to break it. Boiling duration, however, will heavily impact your pasta texture.

Don't forget to salt the water because unlike Asian noodles which usually have salt incorporated into the dough, packaged pasta does not.

I usually use as little water as I can get away with, as the resulting pasta water is much more starchy and works better in sauces.

I hear you all about having nice, starchy pasta water but it is important to have enough water that the pasta can move around freely or else it can end up being cooked unevenly. I think that's probably where the water ratio recommendation comes from.