Can I use a 2016 MacBook pro charger on a 2019 MacBook Pro I’m borrowing a 16 inch 2019 MacBook Pro while my 2016 is in the shop, I figured I could use the same charger being USB-C and it would just charge slow but it doesn’t seem to be charging at all... is it just slow or do I need the other charger?
The wattage requirement of the 2019 MacBook Pro (96W) is greater than the capability of the 2016 charger (87W), so it will either charge slowly, hold its charge level, or continue to lose charge slowly, depending on usage.

Analogy time: imagine a bath half full of water (the battery). The plug is removed, and water leaves the bath (energy used by the laptop). You add water by turning on the tap (Recharging).

If the tap can't supply water fast enough to match the water going out, then the bath level (battery) will continue to fall, but at a slower rate than no water going in at all. 

If the tap can supply only a little bit more than the water going out, then the level will rise, but slowly.

With the laptop shut, it should still gain charge, but a bit slower than usual.