The creature remains intelligent.
The Awaken spell has a duration of "instantaneous", which means that there is no active spell keeping the (now) creature intelligent. It just is.
That means when the creature is Awakened, it immediately becomes a free-willed NPC with intelligence 10, forever.
It then also becomes Charmed towards you for 30 days, which makes it easier to interact with it, but it is already free to do whatever it wants. It could walk away 5 minutes after being Awakened (although I wouldn't generally consider that good DM'ing without a very good reason). It just can't attack you outright while charmed.
The line "stays friendly" suggests that the creature is initially friendly to you, although the spell doesn't seem to state that outright.
Regarding the things you added:
I'll add that a group of druids could with that introduce a new race to the game.
Maybe, but it's up to the DM whether or not the intelligence is hereditary, both when the awakened creature mates with a common animal of its type or a second awakened creature of the same type. (It's also a DM call on whether or not an awakened creature would be comfortable mating with a non-intelligent creature of its same species)
A player could make an NPC out of a pet and so on.
Definitely! That's what the spell is for. That's how Druids make their own friends.