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I'm a little confused on the order of things or what exactly these things are in the ES universe. So is Nirn the galaxy, Mundus the planet as a whole and Tamriel is obviously the continent?

Tamriel is a continent on the planet Nirn on the plane of Mundus.

Planes and planets are the same thing. That's why in books they're labeled "plane[t]"

Mundus is an enclosed area of that is sectioned off from the rest of Oblivion in which Nirn and the Eight exist, each of the Eight being a plane[t].

Simply put: Tamriel is the land, Nirn is the world, and Mundus is physical reality surrounded by the black veil of Oblivion which blocks much of the light of Aetherius behind it (the leaks are the stars and sun).

Mundus is the mundane universe - the Grey Maybe. Nirn and Tamriel are encompassed within Nirn, as well as the two moons Masser and Secunda, and possibly other minor realms and adjacent places. It is a pocket of creation within the wider seas of Aetherius and Oblivion.

Nirn is the central world within Mundus, around which smaller bodies revolve (Masser and Secunda primarily.) If you thought of it in terms of our own world it would be a "planet", and TES does indeed sometimes use this terminology. It's not a perfect comparison, though. Whereas Earth is NOT the center of our universe, Nirn very much is the center of creation in TES.

Tamriel is the main continent of the narrative of TES. All of the games so far have taken place on Tamriel primarily. Other continents, subcontinents and island chains exist and appear in the lore from time to time (Akavir, Yokuda and Aldmeris get mentioned more often than others, but they're three names/places among dozens) but Tamriel is the focal point of the series. Tamriel consisted of ten main Provinces during the Septim Dynasty - Cyrodiil, Orsinium, Skyrim, Hammerfell, High Rock, Valenwood, Summerset Isles, Black Marsh, Elsweyr and Morrowind. At earlier and later times in history the makeup of the land is radically different.