On the surface level Eternum works like your average futuristic VRMMO: the game package comes with a headset, a neural link that supposedly acts to connect the mind and body to the game, and an "optional" bodysuit that supposedly allows one to fully feel the various sensations you experience in the game (sight, taste, touch, the full works).
When you login, your body gets a full scan that copies your entire physical appearance into the game. Physical customization in the game is limited: you are only allowed to customize hairstyle, clothing, accessories, body tattoos, makeup, and the like. You can't change your height, weight, sex, race, or anything along those lines. However, certain servers essentially force players in adopting the "default" style or race of that particular server (so for instance, you find yourself in a mermaid server, then you transform into a mermaid).
Progression is done via a leveling system that acts more like WoW's prestige system: rather than getting stat bonuses, you are instead allowed access to certain items, equipment, and/or spells depending on your level. However, if you die your level is reset and all items in your inventory get lost unless you manage to retrieve them from where you died after the 1 day cooldown period (where you can't login).
Access through the various servers of the game is done in the Nexus, the place you first find yourself in once you log into the game. The Nexus is home to Idriel, an AI construct that serves as your guide to all things Eternum. Aside from providing new players the necessary introductions and rules to the game, she can provide you with hints and suggestions as to where to go next, as well as answer questions pertaining to the game's myriad number of servers (though granted, she can't answer EVERY question, especially with regards to the mcguffins that make up the VRMMO's main quest).
While the VN itself touches on a few of the most popular servers, the VRMMO is said to contain thousands upon thousands of servers, some spanning the size of galaxies, others just being as small as a player-owned beach or an island with a magical school. There are basically 2 types of servers: public servers are vast places where players could come in and out without issue, and private servers which are given to players (either being sold to them or given as a prize to a quest or contest) by the developer from which they can do whatever they want (within reason, of course).
And players are basically allowed to do what they want as long as they don't do anything illegal or against the game's code of conduct (i.e. rape, torture, pedophilia, human trafficking, black market shenanigans). Failure to follow this earns you the ire of the Praetorians - Eternum's AI police - who will demand that the offender cease their activities and log out, their actions pending review. Anyone who fails to comply gets "excommunicated," which is basically automatic account deletion (right after they kill off your avatar) and complete ban from ever entering Eternum.
While the player can play the game completely as a sandbox, there IS a main, "Ready Player One"-esque quest thrusted upon every one of them at the very first login: scattered throughout Eternum are items called the "Ten Gems of Doom," and whoever ends up collecting all of these end up becoming the "Ruler of Eternum" and CEO of Ulysses, the company responsible for developing and publishing the game. This is essentially the secondary main story of the VN (the primary being something that the main character chooses to do following a certain reveal at the end of 0.1).
Anyway, that covers most of the surface-level stuff about the VRMMO. I'd reveal more but I believe those basically count as spoilers that're best uncovered on your own. Hope that helps.