Where does Facebook - Deep Dive data pull from? And how accurate it is with the ios14 update?

The Facebook - Deep Dive pulls raw data directly from Facebook.
The data will be accurate within what FB reports, definitely can't guarantee FB reporting accuracy overall (as mentioned, iOS14 made things a lot less trustworthy). One thing you could explore is the performance of FB on a last-click basis using source/medium values. You can find this data in a couple places including the Orders and Customers Deep Dives using the source_medium filters (to scope the whole page to specific source/medium(s) ), or by looking at the source/medium-specific visualizations.