OwnHammer's
Studio Mix libraries are his latest.
There are 4 libraries:
112-GTR DLX MIX Vintage Speaker Collection
This is a MojoTone Narrow Panel Deluxe open back 1x12 guitar cabinet, with various speakers manufactured before the mid 1990's, using various mics and mixes of those mics.
212-GTR BOG-SH MIX Vintage Speaker Collection
This is a Bogner Shiva open back 2x12 guitar cabinet, with various speakers manufactured before the mid 1990's, using various mics and mixes of those mics.
412-GTR MAR-CB MIX Modern Speaker Collection
This is a late 70's Marshall 1960B 4x12 'checkerboard' grill cloth guitar cabinet, with various speakers manufactured after the mid 1990's, using various mics and mixes of those mics.
412-GTR MAR-CB MIX Vintage Speaker Collection
This is a late 70's Marshall 1960B 4x12 'checkerboard' grill cloth guitar cabinet, with various speakers manufactured before the mid 1990's, using various mics and mixes of those mics.
All libraries contain IRs of a specific speaker in the cabinet, using various single mics, as well a mixes of those mics.
Those mixes are targeted to "studio" use versus "live" use, as explained in the PDF manuals. The microphones which were used are also explained in the PDF manuals.
Also, the IRs / mixed IRs come in 3 variations: SS, T1 and T2. This represents the power amp used to acquire the IRs. (Not to be confused with power amplification in the Amp block) The power amp type influences the tone of the IR slightly.
The OP's question is: Where to start?
As always I recommend taking it easy and simple.
Don't start off mixing IRs yourself.
If you mainly play rock or metal, try the Marshall CB cab library. From one of the V30 libraries (Celestion V30), select the "DYN 57 U3" IR, SS variation, and this should give you a mainstream rock tone. If you want your tone to be focussed on playback through FR amplification (rehearsals, gigs), select a Live or Median Mix IR instead. Or try the SP mixes for a proven recipe.
There are many types of speakers and microphones. One cannot expect Fractal or OwnHammer to explain about them. It would be like asking a car manufacturer to explain all types of available tires.
Here's some information about microphones