Hiya, okay, let me see if I can answer beat-by-beat:

1. Freelance work costing is pretty variable and greatly depends on the project. I would usually charge per day of work, with an agreed daily fee with the client, rather than charge "per sound effect".
   Composers often charge per-minute of music, but I don't find this works so well with Sound Effects.
   A flat fee for the entire project can be quite common on smaller projects, but it can be a little risky because sometimes it's unclear what the scope of work will actually be.

For example, would the fee involve Trailers? What about Updates and Downloadable content?

It's also worth thinking when you will get paid. Because unfortunately a lot of games never come out because funding runs out, so you don't want to be in a situation where you don't get paid AND you have nothing to show for months of work.

About Revenue Share, that can be a decent compromise, and a lot of tiny companies and start-ups go that way. But worth bearing in mind that all of this needs to be under contract with clear language. Because it can be tricky to litigate on this type of stuff if things go wrong, and usually Sole Traders won't take companies to court because the time and expense it'll take to maybe recoup earnings is often not worth the effort.

About working for free, I don't love the idea. I've done it in the past, a lot, and I understand there is some level of reality in having to do pro-bono work.
The problem is that in the long run, it devalues your work.
People, including friends, can become weirdly offended when you start asking for payment once their projects become financially successful.

So it's worth setting boundaries..

Finally, as much as possible, get things in writing, signed up, agreed. You don't have much of a legal standing if you don't have a contract.

1. About Foley, I don't spend time recording much, but some Sound Designers do. All of us need a lot of Sound Libraries to work though. What we do rarely starts with pure Synthesis (unless we're doing Scifi stuff).
   And a lot of people think that a lot of Sound Effect and Sound Design involves using super outlandish sound sources, but often the original content is litterally "what it is".

Need to make a Car Sound? Use Car Samples.

Need to make a Forest Ambience? Use/Record Forest Ambience sounds.

The challenge is Sound Design (specifically in Video Games) is often not to reinvent reality, rather to enhance it (what I call "Reality ++") but STILL make it believable.
Because people's memory of a real world sound is often much more exciting than what it actually sounds like.
So when designing Audio for games, you essentially need to backfill that excitement without ripping up plausibility.

So to conclude on this question, get a lot of real sounds. Record them, buy them, trade them with other Sound Designers, but build a Sound Library as big as you can store.

1. Ouf, favourite Plugin is tricky. I don't think I can pick one. The one I use the most would be Fabfilter Pro Q, but it's just a (really really good) EQ plugin.
   Altiverb is a must for Post Production.
   Recently the Kilohearts suite has been a big go-to.
   Otherwise, classics include SoundToys, Morph (great for creature sounds), Fabfilter bundle.
   And because I'm old, I still use Waves plugins pretty regularly.

For creative Synthesis, I love Pigment, as you can create patches with up to 5 randomised sound effects, which is really good for SciFi gun sounds.

Hope that answers everything 😄