"Bookshelves" are a Nickel concept. There's no direct equivalent.
On the other hand, you must be creating your Bookshelves out of *something* in Calibre, and that something is usually another piece of metadata than we *can* work with.
Since you mentioned tags, that works, as long as you're disciplined in how your Calibre library is tagged, so you're not drowned in a sea of useless tags.
(e.g., back when I actually bothered about Collections (the Kindle's equivalent of Nickel bookshelves), I based them on a set of very specific tags, which where prefixed with a specific character. I still keep those to this day to group books by genre, so I have tags like +Fantasy, +Mystery, etc. And those pop up at the top of our "Browse by tags" results thanks to the magic of alphabetical sorting
).
I've never actually used Bookshelves on Kobo though, because, in practice, I find that they just bog down both Nickel & Kindle, and a plain Search by other metadata is just much faster. (Which means I can't tell whether my "tags with a prefix" idea works with the Kobo driver. It worked on Kindle because the Kindle Collections plugin was very specifically engineered for that kind of trickery).