There is some basic information about using Cubase with a StudioLive mixer in this manual, which you can find on the download page of your mixer: "Using_Your_StudioLive_Mixer_as_an_Audio_Interface_with_Universal_Control_V2_EN_25032019". Section 3.1 is about Cubase.
I only know something about Studio One and I can only recommend to use the "presets" PreSonus supplies to make working with a DAW as easy as possible. Here is a list of steps which can help you to get things going:
1) Reset project and scene (separate) of your mixer and perform a Master Reset of patching,
2) Create a new song in Studio One with the StudioLive 32 Session (the 16 Session is also ok),
3) Now Studio One shows up with all tracks armed and you see signals coming from the mixer to the tracks.
The song template organizes the input and output patching, so if you want to play back the tracks to the mixer:
4) Disarm all tracks in Studio One,
5) Select USB as input source for all channels on the mixer (has do be selected per channel).
6) When you hit "play", you will see signals coming to the channels.
This should basically also work for Cubase and any other DAW.
Using flex/aux mixes is easy. Whatever signal comes in via Analog or USB as input source, is automatically sent to the flex mix. When recording, you will hear the analog input signal, when playing back you will hear the individual tracks. You can also add effects by using the effect returns in each flex mix.
