Just a note of further clarification of signal flow for an effects loop. And speaking in general or usual usage terms. First there are two sections to amplification, the Pre Amp section, and the Power Amp section, these can be in separate boxes one being called a "pre amp" and the other a "power amp" ,,,, OR they can be combined into one box ( this is what you usually see with "guitar amps" )
Either way the Effects loop connections (labeled either send/return or out/in ) sit between the "preamp" section and the "power" amp section. Or can also be said to be, after the preamp and before the power amp.
For a bit more info if your interested . Here is a good brief general run down on the subject ...
https://www.sweetwater.com/insync/use-amps-effects-loop/