I made sure to check if this had already been answered but I could not find one. So my question is: is なんだから related to のだ? Looking at some example sentences it seems to imply something like that but I'm not sure. I can't see to find a definitive answer.
...なんだから is related to のだ through various processes at work here. There is a simple contraction here, i.e. なんだから -> なのだから.
Let's split this phrase up into the individual bits: な+のだ+から.
The な is required before using のだ when the preceeding work is a noun or na-adjective; in other words, in sentences that would end in です (だ). This is similar to how sentences that end with ~と思う require the subphrase to be marked with the copula (i.e. ~だと思います).
Hopefully this will help clear up some ambiguity.
なんだから is an abbreviated form of なのだから, which contains 「のだ」, so in that sense they are the same. But depending on where you see the のだ, the meaning may be different. For example these two sentences have a different usage of のだ: 食べるのだ and 僕のだ