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For starters, subnetting is really tied to Classful IPv4, i.e. it doesn't really apply to IPv6.
Dividing an IPv6 address block, is much like dividing an IPv4 address block.
For example, an /122 can be divided into two /123s, which (each) in turn could be divided into two /124s, which (each) can be divided into two . . .
You can mix and match the subdivided address blocks, as you desire, of course keeping to binary divisions and not overlapping address space.
For example a /122 could be divided down to:
/123
/127
/128
/128
/126
/125
/124
