It’s not always necessary to use USDT. I top up my card accounts with ETH most of the time lately. The fees on this network are quite reasonable compared to what I observed a year ago, and the transactions are fast. Since I keep ETH in my wallet, it just makes sense to use it. Sometimes, I even transfer BTC to my cards it really depends on the situation, and honestly, it doesn’t matter much which crypto I use.
Do I understand correctly that it auto-converts any crypto into what is needed to buy said groceries / other small stuff?
Correct. When you make payment at POS terminal using your crypto card, any eligible cryptocurrency in your account is automatically converted into fiat currency which is the default for your card. This happens if your card balance is insufficient to cover the purchase. The fiat is then converted into the local currency
if it is needed. This entire process occurs in a twinkling of an eye. However, it's often a good idea to convert your crypto into the card’s currency in advance considering whether the market is in a bull or bear phase.