Saw this earlier figured I’d answer. A main is your strongest hero and you’ll usually start off on this hero, but you will switch accordingly based on team comp, map, meta whatever.
A one trick… You will start on a hero and end on that hero, every map you play every comp you are in, any meta regardless.
You are free to play your “main” all match as many maps as possible and sometimes not a single factor will come up that will require you to flex. The outgoing factors and how you responded determine the difference.
You can go entire periods without playing your main, in qp or if Im relaxing in comp I can stay away from Baptiste as many games and maps as I so please, well until the enemy team pisses me off by going pharamercy and I decided yeah I’ve had enough of this.
A person with a main can understand and play the basics of other heros and may also be really good on other heros to the degree of there “main” these are not mutually exclusive.
However if I ask a orisa one trick to go dva they would look at me as if I’m speaking in Japanese. A orisa main also might not play dva but they are either A willing to switch or B have other options.
You can simply “claim” to have a main which is probably the major difference. You can say this or that hero is your “main” a one trick is determined for you by your actions I.e. one tricking it’s literally in the name actually it is THE NAME!
People with mains don’t get bored because we can switch whenever we please with absolutely no limits to how often, how much, or whatever. Even if your not say bad or good you have a “affinity” for that hero. You can also have as many mains as you’d like tho it’s usually to 2 per role, but again you can still play and be good at any other hero of your choosing outside of the ones you claim to “main” (tho at a certain point you just become a extreme like samito levels of flex player but that’s usually reserved for people who are like these overwatch gods).
Now you could have a “affinity” as a one trick as well… Oorrrrrr your just doing a unranked to gm torb only… Your just playing the hero and your already baseline good at the game so having a affinity would be irrelevant. If your say a aggressive player and you play Reinhardt mainly dosen’t matter, on this account all that matters is you know how to play torb and your gonna ride with that hammer all the way to the top.
But yeah thats how it works.