Is Qt3D a part of Qt5?

Question 1

The other answer & comments were correct at the time, but the situation has now changed.

As of Qt5.5, Qt3D is now included as a "technology preview" (I interpret this phrase as an indication that it's not deployment-ready, so there could be some bugs and the API could change).

It's very important to note that the Qt3D that was available for previous versions of Qt has been Qt3D 1.0, while the version included with Qt5.5 is v2.0, which has been developed by a different software team and has a very different API.

More info from the new developers available here.

Note also that Qt3D v1.0 was never actually included 'properly' with Qt in any version, though was (apparently) reasonably easy to build along with Qt if checked out from git from v5.0 through 5.4.

Question 2

Yes. Qt 3D became a standard Qt library in the Qt 5.7 release.

With Qt 5.7, we are bringing in the Qt 3D module. This module has been available as a Technology Preview for two releases now, and I’m happy to announce that it has now reached the state where it is becoming a fully supported member of the Qt family!

This addition was a long time coming. Roughly 4.5 years ago, they announced they were dropping it from the Qt 5.0 release. Since then, it's undergone significant rework. Now, seven minor versions later, it's finally made it back in.