So alien viruses could directly and initially interface with Earth based life-froms cells (humans,the biota inside the humans, etc.), with probably also, somewhat predictable results. Or at least novel coronavirus like outcomes.
Entirely possible, but COV-19's novel spike protein was developed through brute-force trial and error. An alien virus would need to do so via happenstance.
That's just another nice way to describe evolution. It's just a storm of probabilistic biochemistry. I think if you read about how fast and crowded biochemical interactions in the cell are [1], you will quickly appreciate that's it's basically just quadrillions of hypersonic processes doing what you describe as "brute-force trial and error".
For example, that's how a good number of drugs work that attempt to bind to some protein, like Aspirin. They don't go and find it. The drug molecules enter the cell space and just bang around at incredible speeds until they finally just jam into the protein they need to. Brownian motion and similar effects and not some taxi service to the target protein. It's basically just probability which is where concentration comes from. Increasing the probably of the drug molecule hitting the target.
That's also why I really dislike the personification of things. SARS-CoV-2 didn't consciously do anything. I know most people know, but the language constantly insinuates just that.
[1] http://www.righto.com/2011/07/cells-are-very-fast-and-crowde...
It's super unlikely that a virus that evolved outside of Earth would (on its own) have a mechanism to infect cells they hadn't encountered before in order to propagate. Again, nothing is impossible, but physics, chemistry and biology would all have to line up in a very specific way for that virus to infect a human cell.