Overall
This is a brilliant game which unfortunately won't be for everyone. I did feel a little lost at the start, when it seemed like I had a gargantuan task in front of me that I didn't understand how to complete, but that feeling passed after I felt more comfortable with the flow of the game and how to make progress. If you enjoy piecing stories together and the complexity of stories-within-stories and don't mind what some people might perceive as a lack of "gameplay" then you will probably enjoy Immortality. I'd recommend going into it blind and doing as much as you can before you start to look things up as the overarching story is the draw behind the game and spoilers may take some of that satisfaction away from you.

Gameplay
Gameplay in Immortality revolves around playing movie clips and searching within them to unlock more. There aren't "puzzles" as such and there is an element of randomness to unlocking clips which some people might find a bit frustrating. There are little clues built into the game to help you overcome any points where you feel stuck - audio clues or vibration (with a controller) will help if you feel lost and thankfully if you have a controller you don't have to rely on audio clues if you're hard of hearing or find them difficult to listen for.

Endings
You'll get the ending naturally through playing the game, probably long before you finish finding all there is to find. There isn't an extra ending so much as there is the closure of having seen everything the game has to offer.

Achievements
Most of these you'll find naturally as you progress through unlocking the clips. A few will require you to search for specific things within clips. None are missable and it doesn't feel too onerous to get 100%. I'd recommend going as far as you can on your own before you look anything up as spoilers may ruin the experience somewhat, but once you've found all the clips available a guide may help with the last achievement.