I hope I managed to explain it as per your questions.

Nope, this is where we upgrade the cards. Let me show you the exact screenshot where we create our deck by putting our cards in the available slots.

Please click on the collection in the screenshot, here we generate the attack.

This is our deck in the collection area, where we have all our available cards as being displayed.

You may click on the available slot and select one card and then confirm , it will be placed in the slot and you will see the attack above the cards and immediately, it will start mining as per your attacking power or the mining power of the relevant cards.