And then fintech startups like Trade Republic or Just Trade came along and...offered basically free trades (the 1€ at TR can be considered almost non-existent when comparing to the 5-10€ even at discount brokers like Flatex).

They found a third way of financing themselves: just support one marketplace which kicks back some money to them on each trade, which they in turn earn from the spread between buy and sell prices.

I thought the US predecessors of this kind of broker (Robin Hood comes to my mind first) which is still pretty new in Europe would have basically the same business model - earning some money with each trade by scraping off a few cents (or a good bunch of cents for stocks with low liquidity) from the spread?