Placing through hole and SMT on the same side
In general you can't place anything on the solder side in case wave soldering is used (watch Youtube videos of how its done and you'll see why). And wave soldering is the default way to solder through-hole components.
With SMD reflow soldering you can place components on both sides since that process uses an oven. The components are then glued on one side. This makes assembly more expensive too, so it's a last resort when board space is tight.
Various use-cases:

You have only through-hole components single-side. Wave soldering is used, relatively cheap assembly process.

You have only SMD components single-side. Reflow soldering is used, cheapest assembly process.

You have only SMD components but use both sides. Reflow soldering is used, somewhat expensive assembly process.

You have a majority of SMD components but some through-hole. Reflow soldering is used and through-hole components are soldered by hand which is somewhat expensive.
In some cases certain through-hole components can be used during reflow, in which case they are called &quot;through-hole reflow&quot; (THR) or &quot;pin in paste&quot;. This is somewhat new still and you need to check with the assembly contractor before designing in such components.

You have a majority of through-hole components but some SMD. Again likely hand soldering, though the contractor might have some tricks to combine the two, check with them before designing.