Throughout time countries have raised citizen militia. If you haven't got the technology, the time or the resources you just get the people with an odd assortment of weapons that are readily available.
Not really. If you can’t arm citizens with modern rifles, you’re not really going to send them out with Iron Swords and Wooden Shields - you’re not even going to have the technology to make those anymore. You’re certainly not going to send them out with Clubs and Loincloths. More likely would be whatever civilian firearms you have, whatever other makeshift weapons you have, and homemade petrol bombs and or IEDs.
The whole resource thing is a bit wobbly anyway. No one is digging up Niter for muskets - you manufacture the Niter yourself. The real bottleneck for muskets is more likely steel (and therefore Iron). Horses were a geographically limited Resource at various points in time, just ask the Aztecs. But once you get some horses, you can basically breed your own (eg Apache). Although maybe Civ VI does represent that a little - not every horse unit needs horses, sometimes Iron or Steel is the requirement instead.
Anyway. I mostly like the Resource System, although it could use a tweak here or there if only for flavour. But being able to build previous Era units is daft. If you can’t build the unit you want because of Resources, then build AC and Ranged instead and deal with it. Or buy the Resources you need. At most, you should only be able to build one era back.
Separately, I wish Civ had more of a jump from Non-Gunpowder units to Gunpowder. The invention of Gunpowder should feel like more of a jump.
@nzcamel I'd just point out that warriors don't go obsolete until Gunpowder.
I’m not clear when all the various units obsolete. Might be the rules already deal with some of my points above.