Maggie's Farm

flawed analogy time. Viruses are kind of like cars on the highway. Every year new models come out, but they're usually just modifications of the previous year's model with small changes. As long as the changes aren't too big, you can still tell, say, a Corvette from a Porsche 911. Over time, as these models evolve, the cues that identify the models change, so a 2021 911 or Corvette may not look much like the same model from the early 1960s, but you can still see the vestiges of the early models an recognize them.

You can also get cases like the 80s Cavalier/Sunbird/Cimarron, where though they have some differences, the underlying similarities are enough that you can recognize that they are all basically the same car.

Every now and then a new model gets put on the market that you can't recognize, and you have to learn the essential features to identify it against the background of existing cars.

The viral similarity is such that even though some of the surface antigens may differ across variants, there is still some commonality that your immune system can identify and attack. It's also possible to have cross-immunity, as sometimes different viruses share enough of the same surface proteins that immunity against one confers at least some immunity against the other, as in the historical Jenner Cowpox inoculation against Smallpox.