What assignments does an Army (11A) Junior Infantry Officer have within the SBCT and ABCT?

You have staff, CO XO, or PL options. AS3 is supposed to be a CPT. However, you have different PLTs besides rifle, there is also Heavy Weapons PLTs, and second PL time if selected for the scout or mortar PLTs. BCTs have PAO so UPAR is an additional duty that would take up time.

There are no GO's in a BCT so there would be no Aide de Camp.
Those are the most likely assignments for a LT though a LT can serve as a commander or a staff primary / assistant for any staff section depending on unit manning and needs.
The unit MTOE will assign a rank and MOS to each position but the local commander can task organize the unit as needed.
I hope that helps.

I dont know about SBCTs, but in an ABCT, there is an infantry battalion (you'll be mounted), and in each of the armor battalions there is an infantry company. You'll still most likely be an infantry platoon leader at some point.
As far as staff, because there is always some sort of wait, you'll likely be an assistant S something prior to taking a platoon. Technically AS3 is a captains bullet, so the assistant gig there for LTs is the assistant assistant S3.
Dont let anyone tell you being a movement officer is beneath you. Before deployment, the UMO has the most face time with the BC of any LT, which means faster platoon and better rating if you're good.