The overall approach to implement a debit card that is tied to crypto currency is very similar to one that doesn't involve crypto, because Marqeta's platform only deals with fiat currency (USD, CAD, GBP, EUR, etc.). The management of cyrpto assets and wallets is handled by you, upstream and outside of the Marqeta platform. Operating a card program requires that you maintain a certain amount of fiat currency with the issuing bank. Once good funds are available at the issuing bank, the user can spend funds through the debit card. So, there is an implied conversion of crypto to fiat when you allow a user to use their debit card.
With regards to how this all affects your implementation with Marqeta, the webhooks and data you receive will all the be the same as non-crypto business. The transactions happening on the card network will be unchanged, so there are no specific webhooks or APIs that you will need to interact with that deal with crypto.