It's lineageOS + µG + their assortment of default apps, including i.e. their app store thingy.

The last time I used Lineage it was known as CyanogenMod, so I'm not really in a position to perfectly compare the two. But I'll give it a shot.

Lineage includes many more Google components. As an end user this means that you'll for example be able to use the Google location service, using WiFi and bluetooth to identify where your phone is even when you don't have GPS reception. /e/OS depends on a mostly discontinued Mozilla location service, which would have been amazing had it only worked better. So location data for example inside buildings can be a bit spotty.

/e/OS also has a bunch of extra features to protect your privacy if you should wish, such as messing with your location data so that the apps on your phone will have no idea where in the world you're located.

/e/ also comes with a bunch of FOSS alternatives to the default Google apps by default, while i think Lineage might leave it to the users to fill the phone with apps. You can of course uninstall apps from /e/ and install the Google ones should you wish.

As a footnote: Both its developers, @Murena, and #eOS itself ( @e_mydata ) are present on the Fediverse. :)