You should see it in the resolution of the file as it would scale it down. It depends if there is a re-encode chip or if there is a pass-through facility. Your PC card should also tell you what the incoming signal/resolution is.
In theory yes. Some only work at 1080p, depends on the unit and the streaming service you use. They’re like 9 quid though so no real loss if you buy one and it doesnt work for your particulars.
They’re great little boxes just for generally fixing issues with compatibility between things like satellite boxes and various older TVs. The little components do tend to wear out after a while though as they use cheap capacitors and thin traces to chips.
Fricker reviewed a few free ones (and cab/IR vsts) in the past. Look up Emissary and NadIR.