The Olympic "A" standard is the Olympic "A" standard, for every country.
The Olympic "B" standard is the Olympic "B" standard, for every country.
An individual country's standards are almost certainly different, usually lesser, then the Olympic standard. For example, the US "A" and "B" standards needed to compete in our trials are different (usually slower, but can be equal, though by a recent court decision not greater) than the Olympic standard.
If a country has an "A" going to the Olympics, all others from that country, in that event, must also have the "A".
If a country only has a "B", or several "B"s, only 1 "B" can go.
If a country has nobody qualified in any event, they can still send 1 male and 1 female, regardless of performance standards, for 1 individual event. Usually a 3 hour marathoner. Or in the case of Equatorial Guinea in 2000, a swimmer (Eric "The Eel" Moussambani) who almost drowned in his first (and only) heat.