If someone sent you money via an ACH push, they shouldn't hold money for 90 days. If you have pulled the money in via ACH pull. The bank you have pulled from has 90 days to contest that money transfer. Fintech banks will also do holds and have low amount thresholds to mitigate risk. One of the large fintech banks actively advertises $1 million in FDIC insurance but you can only move $50k a day with no way to remove the fraud controls for your account.

Also none of these fintechs are actually banks. They just partnered with a small bank(s) and are still beholden to these banks compliance team. There are strict rules about monitoring bank transfers and reporting suspicious transfers. If your funneling cash through your fintech bank, you'll probably get shutdown with no explanation.