Subpanel feeder cable type to code?
Garages are just the kind of place where you go through panel spaces like, as ThreePhaseEel says, a teenager through Mountain Dew.  240V power tool, boom, 2 spaces. EVSE, boom, 2 spaces. Double-stuff breakers aren't even an option because everything needs GFCI and/or AFCI (as of NEC 2020).  I presume you are once bitten twice shy, and are eyeing a nice 30-space panel so you'll never run out of spaces again. 

We're all for saving money. But panels are the worst place to chintz out. 

Cable/wiring -- you can use NM-B cable, UF-B cable (under NM-B rules), individual conductors in conduit (cable in conduit is legal if your conduit is oversize enough, but usually so hard to pull that you end up calling an electrician).  You cannot use USE cable, but most USE is multi-listed as other types that are legal indoors, so you're all set. 

Do you need a main breaker in the sub if the main panel has a breaker? Indoors.  Well first, the breaker in the main panel is mandatory to protect the cable run.  But yes, since you're in the same building, no main disconnect is required at the sub.  (in outbuildings, you don't need a main breaker, but you do need a main disconnect, and that's the cheapest way to do that). 

Can I use 1/0 Al for 100A? Yeah, you can use it for 125A actually.  The only gotcha with 125A fusing is that most panels have 125A ratings on their bus stabs, which includes both this space, and the space across from it.  So if you have a 100A across from a 15-15 duplex, that's 130A and that's too much.  A 100A can only be across from 25A breakers max.  A 125A can be across from nothing, so you need empty spaces there. 

What you call 2 breakers is actually a 2-pole breaker with handle ties.  It occupies 2 spaces in 2 rows.  So you are concerned with the loads on each row. 

Do they care how many subs I have? Nope.  They care about the ampacity of the loads you have provisioned into each subpanel, as determined by relevant formulas.  They also care about the ampacity of all the provisioned loads being served by the main panel.  But I trust you won't be exceeding that.