@Paddy66 The work on data portability is documented in this Fediverse Enhancement Proposal (FEP):
https://codeberg.org/fediverse/fep/src/branch/main/fep/ef61/fep-ef61.md
You can find the list of implementations there (built by me and @mikedev).
For logging with a single ID into multiple apps, standard ActivityPub Client-to-Server API can be used, but I don't know any production-ready application that utilizes Client-to-Server API in such way.
The alternative is this FEP:
https://codeberg.org/fediverse/fep/src/branch/main/fep/ae97/fep-ae97.md
It's a re-design of Client-to-Server API based on FEP-ef61.