How to open a buy order with 15 different pairs at the same time?

You use OrderSend() and populate the symbol value with the pair name rather than using Symbol(). Do this for each of your pairs . . .

https://docs.mql4.com/trading/OrderSend

OrderSend("AUDUSD", OP_BUY, TradeSize, Ask, 0, 0, 0, "AUDUSD", 123456, 0, CLR_NONE );

OrderSend("AUDCHF", OP_BUY, TradeSize, Ask, 0, 0, 0, "AUDCHF", 123456, 0, CLR_NONE );

OrderSend("AUDJPY", OP_BUY, TradeSize, Ask, 0, 0, 0, "AUDJPY", 123456, 0, CLR_NONE );

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Wrong. Ask is the price to open the CURRENT (chart) symbol. You must change Bid/Ask, High[], Low[], etc. to the ihigh, iLow equivalents.

Then you can not use the tester.

Don't do it. Code the EA to trade the current pair only. Then you put it on all the pairs.

Of course . . . stupid mistake. Could use MarketInfo( symbol,MODE_ASK) instead . . . but as you said it won't work in the Strategy tester.