In Reaper make sure that you work with the RME ASIO driver, see preferences in reaper...

I do not know how you come to the conclusion, that in Reaper its not possible to create a mono track. There is this pulldown menu where you can either make a track mono or stereo, see the two following pictures. But this is something that you could also ask in the reaper forum.

I am not sure why you raised your initial question. Is it really not clear, why a stereo signal has two different levels (L and R)? I really can not understand your question or you asked it in a way that it comes to misunderstandings.
I try to explain again, why a stereo track has different levels L and R:
Because the guy who mixed the track places the different vocals and instruments not in the center, but placed them to the left and to the right, so that the Sound is more structured and has a nice stereo picture / width / dimension. It should be clear that the the total level on L and R is not the same because volume sums up differently on L and on R and not every instrument plays at the same time and with the same intensity.

I hope I could clarify now for you that Reaper DAW of course supports Mono and Stereo channels like every DAW does and why levels are different.

BTW .. assuming you are new to recording and RME in particular I would suggest that you get more knowledge how to use TotalMix FX in the best way for you.

You could have a look at these resources:

A primer that I wrote, it might save you some time: https://www.tonstudio-forum.de/blog/ind … rnal-equi/

Official RME videos very nicely done, doesn't waste any time, they are nicely structured for a certain topic and much to the point without unnecessary bla and redundancy, pure information, very well: https://www.tonstudio-forum.de/blog/ind … al-Videos/

Also the handbook contains a good description how TM FX works and what features it has.

My suggestions is to get a 1st good overview by the RME videos, then go through the steps in my primer and finally read the section in the handbook if you have already a little bit target practice, then its maybe easier to understand, if you know already a few basics.

In reaper it shows up as stereo because the DAW at the end of the day sends everything out to a stereo channel.

In TotalMix FX it will show up at different places.

1. In the Top Row under HW inputs, because there you see the input signal of your Mic at the Mic input of your recording device.

2. In the 2nd Row under SW playback channels, because there you see the stereo output of your DAW

3. In the 3rd Row under HW Outputs as stereo output if you routed the SW playback channel (middle row) to a HW output which is also stereo (your monitors, phones).