Haphazard Guide on Blender Retopology: Tools and Workflow Notes

Does it need to be in a mode? Not specifically unlike in Sculpting or UV Editing or Animation. You just need to be in the Edit Mode.
Set an object as the snapping or the hires to retopo? You don't actually. Any objects other than your low poly mesh can be the "hires". So you need to hide non-relevant mesh. For snapping, just the regular snapping toggle except that your setting is on 1) Enable Face Nearest 2) Disable "Include Active" (Otherwise your low poly mesh will also snap on itself)
There is a PolyBuild tool like Maya's Quad Draw. But it is not well integrated. It does not play well with the Merge Distance Threshold. Might as well use the regular modelling tools.
Click Edge. Ctrl + RMB. Extrudes the edge.
Double Tap G to slide around.
can immediately drag points. normally without the need for f2. same with maya
Grid fill.
create separate points that is not connected. (i.e. Ctrl + RMB)
You can use the ShrinkWrap Modifier to have a better fit, but personally I just do it near the end of the retopo or checking something wrong with the snapping projection. Not the whole retopo process.