How do people keep the flea prevention from getting on the fur? Or get it off the fur without removing it from the skin underneath, since it being on the fur doesn’t help anything.

For topical treatments you part the hair and apply directly to the skin then allow it to dry. Depending on the size of the dog and the brand of treatment you sometimes divide the product into 3 or 4 blobs and part the hair and apply evenly down their back. If some gets on the fur just leave it and allow it to absorb and sink down to the skin. I wouldn’t wipe anything off. Application is never going to be perfect! It’s just about getting the majority where it’s supposed to be! Edible chewable tablets may be a better option if you’re having trouble or if your pup is wiggly and uncooperative.