You mean the trader's platform, rather than the backend of the market itself, right? If that's the case, have a look at https://www.stockfighter.io/ - you can learn some related ideas following their challenges.
just took a look at stockfighter.io, it does a decent job of trying to mimic the stockmarket but there are some thing which would require a little bit of tweaking. if the basic goal is to practice stock trading 'paper trading or demo accounts' will be the best, plus they allow you to program the real terminal robot to check your stratergies. there are several out there if someone want to look, but in my opinion XM trading has the best one yet.
My aim is not to practice stock trading....I already invest in stocks using a platform like'Etrade'....My aim is to develop a trading platform like Etrade, Robinhood etc.....and fill in the missing holes that these platforms contain.....