Running 100 amp line to detached garage
Your choice of 1-1-1-3 Aluminum is adequate. Most Level 2 car EVSEs only draw 30-32A continuous. A few years ago I dropped 2 gage Cu from my panel to my car charger spot. It was way overkill for my 32A ChargePoint, but just a couple years later and we have 40A and 60A EVSEs available. I could put two Tesla Wall Connectors on that wire and only need to upgrade the breaker. I future-proofed that circuit.

NEC Rules for Underground wiring

Under Driveways and Outdoor Parking Areas


Direct burial wire - minimum of 18 inches deep.
Rigid or Intermediate Metal Conduit — 18 inches deep.
PVC without cement encasement — 18 inches deep.


Under Buildings


Direct burial wire in an approved raceway needs to be buried 0 inches.
Rigid or Intermediate Metal Conduit — 0 inches deep.
PVC without cement encasement — 0 inches deep.


Otherwise


Direct burial wire — 24 inches deep.
Rigid or Intermediate Metal Conduit — 6 inches deep.
PVC without cement encasement — 18 inches deep.


Conduit?

Under your house you need to be in an approved raceway. That can be sheathing, if you buy wire that way and run it "in" the walls/floor. Otherwise it must be in conduit.

I wouldn't even consider skipping conduit. I like more protection for my underground wires than dirt and wire insulation. If someone is digging in my yard, I want him to hit the conduit before he runs his shovel or pick into the wire insulation. Since it's my yard, guess who that digger is likely to be?

The conduit fill table says you can get your 1-1-1-3 into 1-1/4". However, I would put them in 2". The cost difference for larger conduit is small and it makes pulling much easier. I'd also pull an extra pull cord in the conduit. Consider it a gift to the future owner of your home, which might be you!

Larger conduit also makes it possible to later pull bigger wires without digging. That garage panel might be the best place to plug in that new 120A mega-EV-charger, whole house battery, generator, and/or 20kW solar array. If you live in PG&amp;E territory, these "someday" off-grid upgrades have recently moved up the priority list. For me, those potential future uses of the garage panel are the main reason to consider larger conductors.

Minor reasons to upsize your conductors is reduced voltage drop and less resistance. Larger conductors means less of the energy that flows through your meter to charge your car is lost heating the wires. Since EV chargers draw continuous loads for hours per day, the parasitic losses add up.