Like everyone else is saying a good diet, low stress, proper sleep, and certain vitamins/minerals are all good for your health. Of course one of probably the biggest killers of your immune system is getting sick (be it COVID or otherwise), so hygiene is important now more then ever.
One other thing I'd point out though, specifically on supplements, is the inherent flaw in statistics. If society thought eating exactly 1 french fry ever day boosts your immune system then everyone trying to boost theirs would probably eat exactly 1 french fry every day -- then those who eat exactly 1 french fry every day would probably appear to have a significantly better immune system than the average because those who are eating 1 fry are doing everything they can to boost their immune system even if the fry itself had no effect.
So I guess my point is more so that I don't know if we really do know what exactly boosts our immune systems, as certain supplements might be overblown just because they are used by the people trying to be healthy (though it gets a bit easier to know what kills our immune systems), so my two cents is that hygiene and prevention is the best way to go about this.