In editing my videos, I've been using different cameras and microphones. All come out bit differently. then, there's the actual volume adjustments on desktops or laptops when editing. What do I look for when setting a volume gain in Kdenlive? I can see the green bar that outputs the volume signal and the intensity. but what do I shoot for to stay consistent? -10? -5? -2? 0? Is there a guide that explains this?

Also, I keep trying to find an audio mixing tool in Kdenlive. I sometimes stumble on it and it pops up, but seem to forget where it is located.

the mixer can be displayed on menu-->view-->audio mixer (if you want to do that with a complete track by track control)

ALSO.. is possible to fix final volume of a project.. just placing a "volume" effect on "MASTER" (click MASTER and after that place "volume" on "master effects" stack )

There is a bug in the audio mixer. I've already had the problem that the video sounds different after rendering it. The problem is that the decimal values in the mixer don't match the values from the slider. As a consequence of that, kdenlive renders the video with the wrong loudness settings. In my case, I could hear my voice very clearly when listening to it in kdenlive, but after the rendering it was sometimes difficult to understand.

Just have a look here: https://invent.kde.org/multimedia/kdenlive/-/issues/792
Hopefully, this bug will be fixed soon.