AI has no penalty for negative gold balance, so it can maintain a huge armies. And at diplomacy screen I have seen some insane debt figures -500, -1000, -4000, etc. So, as soon as AI manages to acquire around 7 cities it starts to unit spam especially if it borders are close enough to someone else. And only "thanks" to a weak tactical AI, you can beat it even if AI army is twice as large as yours.

So, if a player ends with AI neighbors it's a total war till only one remains the master of the continent. Delaying attack makes AI spam more cities, more units and eventually AI starts to mock player on how puny he is and finally attacks.

With isolated start it's just PvE game against barbarians till your cover continent with your cities or spam enough units to do fog busting. Then depending on continent size it's either culture victory or space / diplomatic one.

As soon as player has no more gold to pay per turn expenses, the amount is deducted from science and every turn / every other turn a single random unit get disbanded.

In most maps I have played most of AI had negative balance around Renaissance Era, trading pretty much stops by then and barter of luxury resources begins with occasional sell at fixed price trades. So the only way for AI to get that much gold is thru production micro managing. But I checked AI that have the same amount of cities as I do, mine are mostly improved with Trading Posts, AI's have quite a lot of farms and AI still can maintain a much bigger army.

So I think that AI is either immune to unit disbanding or have much lower unit gold maintenance costs.