Yes, it’s free. And unlike QuackPrep’s "free trial" that requires a credit card, Khan Academy is entirely funded by grants (including from the College Board and ETS).
The platform is just a tool. A hammer doesn’t build a house; the carpenter does. But a high-quality hammer (UWorld, Magoosh, etc.) will make the job a lot less painful than a rusty, quacking one. quackprep alternative