One does not simply become immune from the Covid

Yes, you do. Your immune system discovers the virus and kills it. Once your immune system has built immunity, you are immune. It's literally the definition of being immune.

No you don't, at least not permanently. There may be lifelone immunity for diseases due to bacteria, many viruses, but not Covid-19, a coronavirus. You can get the common cold repeatedly, you can get the flu multiple times. Covid-19 is mutating (on average, two changes per month in its genome) and there is no evidence yet that one can't get Covid-19 a second time, as the duration of immunity is still unknown.

Science has developed vaccines for some viruses (e.g., measles and mumps) but there's yet never been developed any vaccine for a coronavirus -- if science is successful it will be a first.

You don't get the same flu many times. It needs to change into something your body doesn't have immunity against. Thats why there is a different flu shot each year, teach your body the new immunity. If Covid-19 mutates, logic would follow to get it again. But, cases of people re-catching covid-19 are so rare that scientist can't confirm it really happen. It's likely they got a false positive and then later really caught it. Or, cases are so mild that you don't develop anti-bodies. None of this applied to Trump. He had it, tested positive many times, built immunity, recovered and now tests positive for anti-bodies. He is now immune.

It's not a question of if: Covid-19 is mutating, and within even the crowd of several hundred present at the White House Rose Garden a few weeks ago there could have been multiple strains present. That around three dozen people within the White House and Congress apparently caught it there was not remotely surprising given nobody wore masks.

I don't disagree with your basic point, but the fact is that for coronaviruses such as the "common cold", a person can get it more than once even within the same season, even with residual immunity. And as you may know, generally speaking a second viral infection of the same strain usually has much milder symptoms, though the person can still infect others during that time. I could provide references if you like, but a quick search of medical journals on the Web on something like "covid-19 mutations" could provide the same result.

Trump's immunity is current a very politically-charged claim whose veracity is almost entirely unknown. The record of truthfulness from Trump and the White House is so low that there's little reason to believe it, especially given that for the claim to be false would essentially end his election campaign. You can believe it if you like; I for one don't. He could easily still be infectious, and the fact that he has and continues to mix with large numbers of mask-less people means he can't specifically, individually be held responsible for any given infection. But his words, actions and his anti-mask stance have and continue to kill people, regardless of his immunity or lack thereof.