There is not much choices in terms of apps to trade on the stock market in Canada. You may have heard of Robinhood (which is not available in Canada), so let me suggest you an alternative to Robinhood, its called Wealthsimple Trade ! (not to be confused with their Wealthsimple Invest and Save app which is more of a long term savings app)
Wealthsimple Trade let you buy and sell stocks from many exchanges (canadian or american) without any commissions ! The only fee they have is 1.5% on every currency conversions (from CAD to USD and vice-versa). So you if you only trade stocks in your currency, you won't have to pay any fees, ever.
The app itself is very basic, so you might want to also use other platforms to read news and other stuff related to stocks you plan to trade. But for the transactional part, this app is great and works very well !
There is sometimes little lags spike while using the app, but I think having too much stocks in your watchlist is not helping. Other than that, every transactions get treated very quickly (when markets are open).
Overall, it's interface is very clean and minimalist, it makes buying stocks a pleasant experience and I definitely recommend it !