Short answer is that we get on referee ASSIGNORS email lists and/or their platforms and they share their games that are available. We contact the assignor, request the games, and they assign them to us. The pay is sometimes in the email or in the platform. By platform, I mean there are apps or websites that are used like Assignr.
So every internal rec league, travel league (NCSL, ECNL, MLS, GA, etc), and tournament has an assignor hired by the club, league, or tournament to find referees for their games. Some leagues use more than one assignor. For example, NCSL has one assignor, EDP has a few. Seems that each club in EDP has their own assignor. ECNL has one assignor. ECNL-r has at least two. Etc. So once you become certified to referee, its up to you to contact these assignors and ask that they add you to their email lists and then you can add yourself to their group on the Assignr app. I personally am on the list of 6 assignors and it covers my local rec league, NCSL, ECNL, ECNL-R, local EDP, GA, etc. So I log into Assignr and click on that assignor's group and it lists all of their available games. For most, I click on "request game", highlight and copy the game, paste it into an email to the assignor, and request the game(s). You will receive an email confirmation eventually giving you the game. You can then click and see all the games assigned to you. You can toggle between the different assignors and see what is available. As to the fees...they are all pretty similar but some can differ by $5-$10-$20 between leagues. It depends. I keep a spreadsheet with all the fees paid by the different leagues/tournaments. Once you are assigned the game, the pay rate pops up next to your name in Assignr. So this question came up in the Viva tournament thread. I posted that the tournament has not requested referees yet. It is possible that they used an assignor that I am not associated with, but I find that hard to believe. Normally, assignors email a few weeks in advance saying...Hey, Dulles tournament is in 3-4 weeks, I'll be loading the games in Assignr next week. Send me your availability if you want some games. Something like that. But nothing like that has come out for this Viva tournament that I know of...and the tournament is in one or two weeks if I'm not mistaken.
With all that said...just remember that ECNL/MLS/GA/ referees are not better than rec or NCSL refs or anything. Once certified, you can request any game. Now there is some screening by some assignors (you get to know them if you ref alot) but in general, referees are free to request any games they wish. It's up to the assignor to assign it to them. Hope that makes sense.