Your virtual card and your physical Brex card have different numbers, expiration dates, and cvv codes intentionally. The cards are different so that in the event your physical card gets damaged, lost, or stolen, you don't have to worry about changing or updating each of your online subscriptions.
You can use whichever card you would like for online transactions. For in-person transactions, you will need to use your physical Brex card.
Brex advises cardholders not to use their virtual cards for transactions that would potentially require them to present their physical credit card for physical reference. Examples of such transactions include hotels, theater tickets, and car rentals.