He let his private equity / investment company (Alameda) borrow unlimited customer funds via a software backdoor for various investments.

Additionally, they outright lied about things like their reserves. The website had what claimed to be a real time ticker showing the reserve level that was actually a weighted random number. SBF claimed that he fell victim to poor risk management, but the more investigators dug in, the more evidence they found that there was organized corruption.

(and lost most of it by "investing" in shitcoins, basically)

Don't forget the part where they based most of their valuation on a currency they printed.