in short terms, the immune system does it, but it has to learn what it is first.

there are "tags" on the virus, these are antigens.

the immune system samples the antigens presents them to the "antibody librarian" and the librarian publishes a bunch of antibodies that stick to the virus antigen.

This can take some time to get to this state, during this time the virus is replicating making you ill and doing damage.

when enough antibodies are being produced, new copies of the virus will be tagged with the antibody and destroyed with haste.

here is a big read with all the real [complicated] terms:

https://en.wikipedia.org/wiki/Immune_system

this is basically the same process for virus, bacteria, and even some nonliving materials.