I don't know how useful a subsonic slow bomber will be in a fight against top level countries. Low frequency radars can already detect stealthy planes. Even if they can't get weapons grade tracking, just discovering a slow plane gives you time to scramble fighter jets to send against such slow planes. So, once they are detected, they have no defense against fighter jets.
So, the rumor that China is prioritizing a super sonic JH-X over the H-20 is probably the better approach to take.
If China has low frequency Anti-Stealth Radars setup all over the country. The only thing B21 will be good for is firing long range cruise missiles from a distance. Cause if they get too close, they will be detected by low frequency radars and fighter jets will be sent to destory them. So, this kind of long range cruise missile can be already be done by H-6 bombers.
Compare to that, a JH-X with supersonic speeds can not only fire those long range cruise missiles, but also fight back with A/A missile against incoming fighters and also flee fast or evade incoming missiles with its supersonic speeds.
So, no. B21 is not a game changer against China and China should not be desperate to counter it with its own similar bomber. It should look at the trajectory of technology and select the right cruise missile platform.
The fist photo in this series of four contains a better picture of those flaps along with looks like a sort of inlet directly towards the rear of the aircraft? It looks like something folds inward (towards the centerline of the B-21) to create that additional inlet.
EDIT: I also think you can see the pilot in the first first photo in this series as well.