Keep finding hornets in house wonder how they get in and where do they build?

They usually get in underneith your siding near your roof and get into your attic. Knock all of the nest down that you can find and see if they are getting through your attic because they like to nest where it is usually warmer of a night.

If your home is modern, it probably is pretty well sealed. However, there has to be points at which your HVAC system vents. A common place for insects to get in is through the vents in the attic. Even air conditioned houses often have vents to let out hot air. Sometimes, these are unprotected or damaged. You can prevent the hornets from getting in with mesh which can be nailed on the inside of the vent, which often is under an eave at the peak of the roof of the attic.

A screened window with a hole in the screen. They usually build nests near the ceiling

There is probably a base nest inside that you have not found yet.

they probably get in from a hole in the window or in the walls

you can try getting them out and then you can look for a bug place and they can spray it for you!