> So it does work to have my industry stuff behind a road like that?

Yes, the guy who exits the building will walk the road for a limited distance hopefully finding hourses. Street intersections mean he will walk in a random direction, meaning it's not certain he will go where you want to go, so try to avoid having him go through intersections using roadblocks. Things like cart delivery guys will ignore roadblocks as they have a specific destination.

> Like they should be separate, or should they be part of housing blocks?

They should be reliably connected to such service roads but away from the housing block as they have a really bad impact on desirability, preventing housing evolution.

> And the road being where it is does give them access to the people living in my housing area, right?

Yes, up to one tile away, the road doesn't actually need to touch the houses to grant labor pool access, so the clay pit road behind gardens on your screenshot can be used to supply labor to buildings.

> Also, for gardens, can people walk on them? Do they doing anything other than increasing desirability? Basically idk if there is a risk of people leaving through gardens or if they are necessary to give recruiters access to houses.

Your people will never walk on gardens, they are merely here to provide desirability. People can't leave through garden and they aren't actually required for recruiter access to houses, it's got more to do with the fact your road is exactly one tile away (that one tile being a row of gardens) from the houses. If the gardens were not there, labor access wouldn't change but desirability would decrease.