In this picture the connection between panels is PVC, the "Grounded" conductor would apply the connections required in this setup.
When assured bonding is required for example on Ess. and Cons. KO's it would be required to use the methods you have stated....either a bonding wedge or ring for example.....in other cases a bonding busing is used.
Here…this might help explain it better
**(B) Method of Bonding at the Service. **Electrical continuity
at service equipment, service raceways, and service
conductor enclosures shall be ensured by one of the following
(1) Bonding equipment to the grounded service conductor
in a manner provided in 250.8
(2) Connections utilizing threaded couplings or threaded
bosses on enclosures where made up wrenchtight
(3) Threadless couplings and connectors where made up
tight for metal raceways and metal-clad cables
(4) Other approved devices, such as bonding-type locknuts
Bonding jumpers meeting the other requirements of this
article shall be used around concentric or eccentric knockouts
that are punched or otherwise formed so as to impair
the electrical connection to ground. Standard locknuts or
bushings shall not be the sole means for the bonding required
by this section.
But without going into too much detail in the example you show here the panel to the left is backfed, it should have it's own support ( unless it is bolted type and I dont think it is ) holding it on......in addition to it's current support methods.
I am curious however…how come they did not simply terminate on the MLO terminals…it is clear that if this is a service it is under the 6 throw allowance…
anyway…hope it’s helpful