You get push notifications for stocks making big moves up or down. They show you popular lists which curates stocks in various ways (100 Most Popular, Upcoming Earnings, Top Movers), which makes it find stocks you might want to trade. They show you what analysts are staying about a stock (X% say Buy, Y% say Sell). They integrate news articles and autoplaying playing videos with finance news. The graph of you investing performance is animated the same as a game's high score screen. They also give you a free stock when you sign up or for inviting friends.

These are all mechanics pulled from social media and games to drive engagement. Vanguard is definitely not doing any of these things. If I'm more engaged with the app, I feel like I have a better understanding of the market. If I have a better understanding of the market, I feel more confident day trading.

At first glance its easy to view Robinhood as just a better UX for a finance app, but it's clear they've gone beyond just making it easy to buy stocks/options and are actively seeking to drive engagement.

This combined with the rise of things like r/WallStreetBets has definitely inspired a new type of investor/trader.