A buy Limit order allows the trader to specify a price and the amount they would like to buy. The order does get filled only at or below the limit price.
E.g. With the trading pair of BTC/USDT, the current price for 1 BTC is 6822.03. The buy limit is set at 6800. Therefore, the price must drop to 6800(or below) to fill the buy order.
Once you click on exchange buy, the order will be submitted and then reflect in your active orders, unless an entity agrees to sell at that price, or you choose to cancel the order.