Viruses select nothing.

A random mutation happen in a virus genome.

if this new mutation doesn't give the virus any advantage compared to the other viruses => this mutation won't be spread since there are far more viruses with the "wild type" genotype.

if this new mutation give the virus an advantage (infectious advantage, hide immune system advantage, cell control advantage, speed of replication advantage....) => this mutation will spread for the next generations of viruses since this virus has something the wild type viruses don't have that makes him more efficient in his "life" cycle.

All mutations are neutral until the context gives the holder of this mutation a positive or negative selection.

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Living cells are "factories"for viral replication, and living cells are limited resources.

Hence viruses would be at an advantage if they are able to "control" more hosts by replicating more efficiently and/or modify the hosts' behaviour.