"looks" is almost exclusively used as a verb to describe the action of looking when it is done by a single subject. It can also be used in the same way that the word "appears" is used when it is not describing the action of spontaneous arrival. e.g. He looks for the ball.

Is looks singular or plural?

'Looks' as a noun is only ever plural. it can be singular.

Is it look good or looks good?

A past tense verb does not change for person or number, therefore they are both correct. She looks good. They look good. These verbs are being used correctly in the present tense.