The UFP has a currency - Federation Credits. The average citizen may not need (or even have) money, but it exists and is presumably accessible in some way if you have genuine need. For example the station personnel on DS9 never had issues paying at Quark’s, so one can assume that they did get a salary because they were posted to a capitalist environment. I’d assume that child support would work on a similar “as needed” basis.
Usually Steam. I like the idea of GoG, but a lot of the time if you want mods you’re basically forced to buy it on Steam because of the Workshop. Also, I kinda like having everything in one place.