Can I ask which are the 4 big firms?
Did you research accountancy at all before starting AAT?
The "big 4" are the four large audit firms which dominate globally. PwC, Deloitte, Kpmg and EY. They provide audit, tax and consulting services, and increasingly some technology stuff too.
There are then a next tier of firms, still large, but more uk focussed, including grant thornton and BDO, saffery champness, rsm, evelyn, mazars etc. Again, audit & tax are key focuses.
Below this you have small often local providers of audit, tax & bookkeeping. More accounts prep eg for small businesses at this level.
Then you have accountancy services within corporates, where you'll find roles like AR/AP, financial controllers, FP& A (financial planning and analysis), reportimg, consolidation, internal audit. Bigger corporates will have dedicated teams for tax and treasury, many of whom will also be from accountancy training.
Big 4 tend to take high calibre graduates with top a level grades, who will study for the most prestigious qualifications - ACA, CTA, they also more recently have apprentice schemes for school leavers but they are cherry picking bright high achievers.
2nd tier firms will also take mostly graduates and focus on ACA and CTA.
Smaller providers will often have people training for aat, att, acca, sometimes aca & cta as well.
Corporate finance teams often have trainees doing CIMA, with a few of the other qualifications above.