does the air bnb host usually stay with you?

It varies depending on the property. Is the person advertising a room or room & bath, or an entire apartment, or a house? Study the listing carefully, because it will state! E.g. if the headline reads something like "Queen bedroom with en suite" (or queen room with bath), that's the spare room in someone's house or apartment. The owner will specify kitchen privileges, amenities available to the guest, etc. 

Read the listing.  If you are renting an entire house, then they won't be staying with you.  If you are renting a room in a house where others rent other rooms, then the owner probably isn't there.  If only one room is available for rent and the rest of the house isn't, the owner is probably staying in the rest of the house.

no, unless the property is multi-unit.

It could go either way.  The more expensive, the more likely the owner will not be there in the building with you.

Just you. You're not paying to be watched over. Some hosts have been caught out using spy cameras though, mainly for voyeuristic reasons. I read of Airbnb rentals being trashed by partgoers, so obviously the host wasn't there.