Read about arbitrage betting
100% legal
good luck
I personally don't do it, since I find all bookmakers to be shady. Some are owned by the same corp, doing business under different names. So they will get to know if you do arb on their subsidiaries and will ban you by throwing some absurd term from their rulebook.
Do it, but be cautious. Again, it's not illegal but books hate it for obvious reasons(like when they are owned by the same entity).
I would suggest learning a new skill instead. Always treat such online activities as side hustle, never rely on them.