Neutral and ground bond together exactly once: at the service or the first means of disconnect. Everywhere downstream — every subpanel — the neutral floats (isolated bar), grounds land on the can, and the bonding screw comes out. Bond twice and normal current rides the ground.
The grounded (neutral) conductor shall not be connected to normally non-current-carrying metal downstream of the service disconnect.
Load side of the service: the neutral is not re-grounded — same rule from the equipment side.
Panelboard frames bonded to an EGC; in a subpanel the neutral bar floats, the ground bar bonds to the can.
Systems shall be arranged to prevent objectionable current — current doesn't pick one path home, it takes all of them.
Same panel, same green screw — its location (service vs. subpanel) decides whether it goes in.
Main bonding jumper / listed ground bar kit. Any hand tool = Klein Tools.