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It is decentralized. The holder provides the data, which was ultimately provided to them by the government, they're the client. The verifier is the entity that wants to know how old the holder is, the server.

The form are eg things like the JSON Web Token (JWT), Digital Credentials, and the Federated Credential Management API (FedCM).[1][2][3][4][5] The software can be anything since they're expected to use open protocols, so yes, web browsers.[6] Per the Commission, "For remote presentation flows, … the Wallet Instance implements the OpenID for Verifiable Presentation protocol OpenID4VP in combination with the W3C Digital Credentials API."[7]

[1] https://en.wikipedia.org/wiki/JSON_Web_Token

[2] https://github.com/w3c-fedid/digital-credentials

[3] https://w3c-fedid.github.io/digital-credentials/

[4] https://github.com/w3c-fedid/FedCM

[5] https://w3c-fedid.github.io/FedCM/

[6] https://github.com/w3c-fedid/FedCM/blob/main/explorations/HO...

[7] https://eu-digital-identity-wallet.github.io/eudi-doc-archit...