A previous review thoroughly describes the main characters so there's no need to go over the same ground. The stock market crash was exacerbated.by speculators buying shares they couldn't afford on credit. Some investment funds were too highly geared, and downright criminality such as at the Union Industrial Bank played a part. However I contend that buying the likes of General Motors with your own money in 1929 was not obviously foolish, just rotten timing. It is a conceit of our times that we imagine ourselves wiser than previous generations. Maybe it was ever thus. Many 21st century folks apparently see nothing stupid about investing in Bitcoin, which pays no dividend, has no physical use like gold, and has no intrinsic value.
I have a copy of the book on which the film is based, along with J K Galbraith's pithier The Great Crash 1929. TDTBB will appeal to those interested in people while The Great Crash is a more dry, witty analysis of the 1920s stock market. My advice is see the film but read Galbraith.