should the link destination be a page or subdirectory

I'm about to get a high quality link and I was considering having it link to the sub domain in the niche I want opposed to the root directory.

Would it be best to link it to the sub directory, root or the specific page that gets me the most volume?

https://site.com/subdirectory
or

https://site.com/subdirectory/page-I-want-to-link-to

https:// site . com / subdirectory is NOT a sub-domain. A sub-domain is something like https:// subdomain . site . com

Subdomain means https:// subdomain . site . com while root means https:// site . com

So, do you want to link to the sub-domain or to the root? Or do you want to link to the subdirectory? Or do you want to link to an inner page like this one https://site.com/subdirectory/page-I-want-to-link-to ?

Just link to whichever URL you want to rank in google (but I assume that it will be the inner page)

Which one is currently in the serp,

site.com/subdirectory
or
site.com/subdirectory/page-I-want-to-link-to

If the page was in serp for your targeted keyword, then link the page instead of subdirectory.

It depends on a lot in your linking profile.
But if it's a directed content that fits perfectly to the page, link it to the page, it will benefit both the domain and the page, which will get also boosted in case you are interested in that.
Unless you have a great content in the subdirectory, I won't link there. Most of the times i leave my subdirectories with thin content in ******** for interlinking purposes, but noindex.