Yes, it's legal to smoke inside an apartment, so long as your lease doesn't disallow it for example.
Even for complexes where it's disallowed, good luck getting it enforced. I get weed smoke coming in through the ventilation system from who knows where in my completely non-smoking building on a regular basis.
Actually was the same before legalization too, and hasn't really increased, so this is sort of a non sequitur.
If your landlord fails to enforce the lease agreement, you can probably use that to get out of the lease. Our complex recently had people complaining about this very thing and they have started getting much stricter about it thank goodness.
You probably can't, unless your agreement expressly obliges the landlord to enforce the rule against other people rather merely than requiring you to follow the rule as a condition of being permitted to rent the apartment. This would be an unusual lease agreement.
In Colorado, many communities have started to actually enforce this, in the same way that repeated noise violations can lead to eviction or non-renewal. Now that it's no longer a police issue, and just an "inconsiderate prick" issue, property managers can address it in house.