Welcome to the series! There's definitely a simple charm to the game, but I love that you can build upon it with some more advanced techniques (which I'll leave you to discover on your own). The games can be completed pretty normally, but there are some secret locations and shortcuts you can access with a bit of finagling.
This community has been extremely dedicated for the past 30 years (happy birthday, Infinity!), so there are countless different mods and custom scenarios to go through if you want. I've also been greatly enjoying the modern Marathon game that came out recently, even though it's a completely different format. It's awesome seeing a universe I grew up with completely reimagined but with much of the same lore and intrigue.