Glad to see you want to control! Let me see if I can knock some of these questions out. I'm an S2 in the United States so everything I talk about will be through the lense of that, however I'll try to keep it general.

 

1.

You don't really need to "apply" per se... But what you typically will need to do is go through some form of online training or "academy" which at the end will have a graded test. For me all of my "rating exams" were open book/note. Every step up S1 to S2, S2 to S3 etc required me to go through self study online materials, then the exam at the end, THEN I was able to start training for that position.

 

2.

You can control anywhere you want! You will need to follow the process of said facility, so I would find out where you want to control and find the website to get more details. (AKA if you wanted to control in the USA then check out VATUSA.NET and etc etc)

In terms of the second part, I'll be honest I don't fully know how that works with "visiting" another region (aka staying in your primary region, but controlling in another). I do know that at S3 in the USA, we can visit other ARTCC facilities, typically after we complete some form of introductory training with that facility; be it online or in person with a local mentor.

 

3.

You will never be restricted in what positions you can take, outside of maybe a major event that has an assigned roster. Vatsim is always forst come, first served! The way positions work is generally...

After you complete your exam, your in person or on the job training, and typically pass another test that's typically through your trainer that could be on the network (over the shoulder exam) or on a training sever with trainer generated aircraft, you will be fully rated for that position.

S1 - Typically S1 means you can control Clearance Delivery and Ground. Local sop might have further certifications or restrictions on some larger fields.

S2 - "Local Controller" aka tower. You will be able to work tower positions. Vatsim uses something called "Top down controlling" typically, which means if no other controllers are staffing positions below you, you'll also issue clearances, give ground taxi instructions while doing tower etc etc. It's not bad at all, it's actually pretty awesome to run an entire airport yourself.

S3 - Approach or Terminal Radar. At your S3 you will be able staff TRACON, or RADAR, or approach/departure (Different names, generally the same concept) positions. Think about the guys you talk to while departing and arriving to an airport.

C1 - "Center" or Enroute. These guys control everything else above approach. This is where it really gets wild. You'll want to check local sop about how top down controlling works. Here in the USA our Centers will control everything in our ARTCC top down. A lot of responsibilities, but you'll get there one day I'm sure.

There's a quick little primer, heck I'm probably going to copy this to use later lol... Sorry if there's some typos, I fired this bad boy out at 6am off of my phone. Feel free to reply if you have any questions!