Rex: Another Island

The normal ending door can be found in the airship, which is all the way to the right in the world. You don't need any collectibles to open it. After the normal ending you can continue playing to get the other endings as well.

Just keep following the coins and you will eventually find the magic gems as well. After collecting all 5 magic gems, you can open the door that gives you the magic gem ending. The door is located right above the Empty Town teleporter, you can get there by jumping on the clouds.

You can view your coin progress in the top right corner of the screen, alternatively you can press the zoom out button to see in which areas you still need coins.

Although the achievement description says "Find all 777 coins", collecting all 777 coins isn't enough. You also have to find the door that leads to the secret 777 ending. This door is located right above the Forest teleporter. Teleport to Forest, you see 2 birds moving up and down to the right of you. Jump on the first bird and double jump to the platform to the right without hitting the second bird. Now jump to the left on the second bird and double jump up to the tree. On top of the tree you notice that rain is falling on some hidden blocks, proceed to jump up these hidden blocks and you'll find the 777 ending door there.