Ally is the old General Motors financing arm (GMAC) reborn in 2010 as a branchless digital bank. It is the largest all-digital US bank by retail deposits (~$143B) and one of the biggest auto lenders — it buys the loans car buyers take out at ~22,000 dealerships, funds them cheaply with online savings deposits, and pockets the spread. It has been shedding non-core lines (point-of-sale lending, credit cards, direct mortgage) to focus on auto finance, insurance, corporate finance and deposits.
In simple terms
Money flows in as online savings deposits, gets lent out as car loans that people take at the dealership, and comes back as loan repayments plus interest. Ally is the bank in the middle that never had branches — it runs the whole thing through an app.