Hello @johnbury,
I will try best to answer the few questions you have, but someone with more knowledge might chime in as well.
Please see this Adjusting volumes post in the wiki and this forum post for a little diagram how gain is handled inside the duo.
It is common to set a gain or compressor plugin with adjustable volume as the last plugin in your pedalboard to avoid too much digital gain before digital to analog conversion.
As stated in the wiki the mod duo has a true bypass https://wiki.moddevices.com/wiki/MOD_Duo#Inputs_and_outputs.
Feel free to use the search function within this forum and to ask upcoming questions.
Hope that helps and God bless
Marius