Thank you for all you precious advice.
I used Monzo once abroad to take some cash ( Germany) and the ATM machine DID NOT ask for conversion or stated any fees.
However, when I saw the notification on the phone I realised that I was charged a bit more. I contacted Monzo’s customer service and they confirmed that a fee had been paid, due to the ATM policy and not to Monzo.
What I was asking is : when I take money from an ATM, will the app tell me if a fee as been applied? in such case, next time I need cash I will try to avoid that ATM and try another one :).