What *is* the game loop?

A friend's been on the fence about the game and I've been trying to sum up what the base game play actually is.

She likes Arc Raiders, Tarkov, and a couple other games in the genre but whenever I sit down to compare Marathon to any similar games, what makes it fun really diverges from the rest.

In Arc Raiders, the appeal is in the loot (obviously) and the PvE stuff with the occasional run in with another Player. Marathon isn't really focused on hard PvE stuff (Cryo kinda counts just from the sheer volume of bots you have to blast through) and there is way more PvP that Raiders.

The comparison to Tarkov has the opposite effect: that game is really slow, methodical, your team feels extremely isolated and the maps are massive. Marathon's maps are much more compact and often the best gear is behind a sort of puzzle (until you get the hang of it at least).

Ground loot isn't what you're after in Marathon, PvP is a focus but also so are solving cryptic mechanics, and the most engaging part of the game is a multi-step process of finding a key to open a door to then press a button which opens another door where you fight a boss and then get the good stuff.

Help me out here. I have a ripe convert who might come play the game but every time I try to encapsulate what the actual game play is, it just sounds like spaghetti.