I was writing my thoughts and put it like that at first:
An explanation what is the foundation of this stuff. (&)
Then I thought: "Wait, shouldn't is be at the end?", so I rephrased the sentence and looked at it again:
An explanation what the foundation of this stuff is. (#)
As to a non-native English user, this sentence felt somewhat awkward to me, so I've found an article about it:
What as a pronoun
We can use what as a pronoun to mean ‘the thing(s) that’:
What we need to do is make a list of useful phone numbers. (the thing we need to do)
And indeed, my sentence can be rephrased using this rule:
An explanation of the thing that is the foundation of this stuff.
I've also tried to replace what by some contextual synonyms:
An explanation what [inhabits]/[amounts to] the foundation of this stuff.
Or thinking up similar sentence structures:
I'm analyzing what is there.
But I'm still not sure.
According to my understanding of English, the (#) version is totally correct, but after all this research, I'm still not sure about the (&) version.
My question - is the (&) version correct?