That suggests your meter main is rated 125A, and you could confirm that by checking the label. Then if that is the case, and if the service conductors on the line side of the meter are sufficiently large (overhead or underground service?), then it would be possible to upgrade your service from 100A to 125A. I think that requires notifying PG&E, which could lead to them deciding their service conductors need to be upsized, which could be expensive for you if it's an underground service. On the other hand, they may have treated it as a 125A service already, if that's the equipment rating.
To take advantage of the 125A service, you could upsize the breaker from 100A to 125A, if your existing Al feeder conductors are at least #1/0, or if you set a new panel next to your panel to house a smaller breaker for them. Or if the manufacturer of that panel makes plug-on sub feed lugs without OCPD, then you could use the upper bus stab to extend your service conductors to a 2nd service disconnect right next to your meter main.
Having said all that, in practice the 100A breaker is unlikely to trip due to the new load of the EVSE, particularly if you are charging at night. You are, however, supposed to do a load calculation for the entire house to determine if 100A is sufficient or not.
As to the last photo with the (?) AWG markings, if you stick with the single 100A breaker, then the minimum ampacity for the short segment on the left is 83A. For the new conductors to the 50A disconnect, the minimum ampacity is 50A if you use a raceway method (a requirement of the tap rules) or 83A if you use a cable method. For copper conductors, 75C insulation, 83A means #4 Cu, and 50A means #8 Cu. You can, of course, use large conductors.
Cheers, Wayne