The game has 3 currencies:
This is the generic in-game currency. All buildings, salaries and normal players are bought through this currency. Users get these through playing matches, sponsors, winning competitions, selling merchandise, food and selling players. This currency can't be bought and only comes from in-game activities.
In the top right corner it's the amount inside the white square.
This is the premium currency inside the game. This currency is used to buy packs, cosmetics, reduce time of constructions and mostly features to make your life as a manager easier. This currency is bought using paypal or one of the available credit cards.
The third currency is FIAT and Crypto. FIAT as normal money such as Euros and Crypto as in Ethereum or Bitcoin. These will only be applied to make blockchain purchases of plots of land and premium players. Through MetaMask this process is really easy as if you want to pay with for example USDollars you will click on buy and then you will be redirected to MetaMask and then you can pay with your normal credit card or even with paypal. If your intention is to buy with a cryptocurrency then once you get to MetaMask you just choose to pay with that curency and the purchase will be done.