In Solid the primary copy of your data lives in a neutral server and multiple apps can access it. In theory, since Solid isn't really deployed and major apps will never be willing to adopt it.
With data portability you can export data from one app and import it into another but there's no ongoing sync.
solid is dumb. and something that only makes sense for a comp sci from the 60s. everyone else who reads the project in simple english will see how dumb it is today.
in simple english: It is the dream project of whoever come up with cookies. basically cookies as first party data that you can download, upload, shared. All while having either the trouble of hosting a lot of infrastructure (just like the creators of email protocol thought everyone would do, ha!) or relaying all that info to a 3rd party like google or facebook. The nightmare scenario to everyone saying 3rd party cookies are bad.
[0] https://solidproject.org/