The best projects/rates are through your network if your network is in a rich country. For someone who is in an average country, and even more so for a teenager, upwork and similar places can be very useful. Definitely were for me when I was a teen.

Regarding the rates being low on sites like this. Sure there are low rate jobs, and a bunch of people who compete by lowering rates. That doesn't mean you have to do that. It's completely possible to ask for more money on upwork & friends. The classic patio11 advice of charging more works just as fine on upwork as it does elsewhere.