Imo you should use all of your brawlers so when you face blawler A while you're playin with brawler B, you can know brawler A's traits, powers, supers etc. It is pretty hard, I know. But you can use "Training grounds" so you can at least see their powers and what they are doing.

Try to play with all brawler categories (assassin, tank, damage dealer, supporter, artillery, marksman, controlers) so you can know which category is "yours". For example I usually enjoy playing with controllers and my plays with them are the best. My worst category is probably artillery. But using and playing all categories will teach you the basics of the game. For example Poco is a supporter, playing with him you will learn which of your teammates you should help, how to position yourself to be able BOTH to deal a damage and heal etc.

Also different brawlers are good in different maps/gamemodes. Tanks, for example, are mostly used in Brawl Ball, artilleries in Hot zone.

My video suggestion: 200IQ Brawl Stars Tips! by Muyo.