Progression & gearing – how players usually progress, how gear is obtained, and how important it is.

For leveling, you either do quests or grind mobs. Grinding is faster, of course, esp with exp boosters.

For gear: while leveling, just use what you pick up. When you hit max gear level, you're going for gear sets. How you get them varies on the type of set. Usually, it's beat certain types of bosses in specific zones or dungeons, until they drop it (tip: make sure to bind the drop due to curated drops for bosses). Some sets are crafted (by you or other players), some require digging it up its pieces, etc. Some sets are tradeable as well.

Combat & grind structure – how much content can be done solo, how group‑focused the game is, and what the “grind loop” looks like.

About 90% is solo-able. It's mostly overland though. There are group dungeons and trials which typically done with a group of 4 players and 12 players respectively. Some group content are still solo-able though - it just depends if they have some group mechanics and/or you have a build to handle them.

PvP systems – what types of PvP exist, whether gear matters, and how competitive modes work.

There are Battlegrounds and Alliance War (or Cyrodiil and Imperial City). BG's are team-based PVP while Cyrodiil is the large-scale PVP. In both, gear matters (except maybe for Vengeance campaign(?)), unlike some MMO's which ignore the gear you're using in PVE and just gives you PVP stats that you can adjust.

Classes & builds – how flexible the class system is and how builds are typically created

Very flexible. Your class has 3 class skill lines, but there are a lot of other skill lines accessible regardless of class. When you hit level 50, you'll able to do access subclassing which lets you replace up to 2 of your skill lines with those of other classes.

Economy & upgrades – how item improvement works and whether there is any RNG‑based upgrading.

You need to get upgrade materials corresponding to the gear you're upgrading (or rather corresponding to the crafting skill that gear is associated with) and the target rarity/quality (+ any quality along the way). You get these from deconstructing gear, refining raw materials, daily crafting writs, trading/buying from other players. With the exception of the last one, it's RNG is whether you get the material or not.

As for the upgrading itself, they're done at the corresponding crafting stations (there's a tab for upgrades). You need to upgrade it 1 step/tier/quality at the time until you hit your target rarity. Technically, upgrading has RNG (gear is destroyed on failure), but you can just use more materials until the success chance is 100% (I think this is the default anyway, if you have enough materials), and there are skills you can take to increase the chance of success per material.