of which

[capcap23]

now in another universe where B is correct: the combinations are AC, AD, CD of which CD is useless. so again 1/3. A comment from a video. source
Another example: Only a very small number of languages, of which the best known is Yoruba, have pronoun retention as their sole grammatical type of relative clause. source
Can you explain the usage "of which" in terms of grammar?

My assumption is that first one is closer to determiner as in"in which case" whereas the latter can be rearrenged as "the best known of which" although I only saw this type of relative clause used after full sentence.
I came across many sentences like above but couldn't find a source that explains this kind of relative clause usage.

 

[jutfrank]

How much explanation do you require? This is a use of a relative clause.

If you're struggling with the meaning/use of the preposition of, look at it like this:

Of the three combinations AC, AD, and CD, CD is useless. One of those three is useless.

Does that help?

 

[Phaedrus]

the combinations are AC, AD, CD of which CD is useless. [. . .]

My assumption is that first one is closer to determiner as in"in which case"

That relative clause doesn't work. It needs to be set off with a comma, and an appropriate noun phrase needs to complement "which":

The combinations are AC, AD, and CD, of which combinations CD is useless.

There, "which" is a determiner. The meaning of the relative clause "of which combinations CD is useless" is that of the sentence "Of those combinations CD is useless."