How do Phagocytes attack pathogens if they don't have memory? How do they know what is foreign and what is not

How do they tell if a thing is a virus? Especially virus, that are creatures that change "appereance" all the time?

Phagocytes engulf and destroy pathogenic cells. When a pathogen invades the body, it is recognised as foreign by the chemical markers on its outer membrane - these markers are called antigens. Antigens are specific to the organism. Our own cells have antigens, but these are recognised as our own and do not produce a response.

Proteins in the blood, called antibodies, attach to foreign antigens. Phagocytes have membrane-bound proteins that act as receptors. The receptor binds to the antibodies already attached to the pathogen. This process may be assisted by other proteins called opsonins.

Once the phagocyte is bound to the pathogen, phagocytosis (the engulfing and digestion of the pathogen inside the phagocyte) can take place.

I'm not quite sure what you mean specifically about the 'changing appearance' of a virus, but I hope this answer is of some use nevertheless.