Doom: Eternal is a high octane ride that keeps you going through every level of the game. However, the problem with it is you can easily miss some of the collectibles hidden throughout the game. To make it easier, you have access to a fast travel system to hop around the map.
Unfortunately, you can only access this fast travel system after you have beaten a level and reached the end. At the end, right before you complete the mission, a pop-up notification will appear on your HUD informing you that the fast travel system is now online and you can freely use it when you pull up the map. You can then use it to jump around to the different areas, grab any collectibles you may have missed, and then complete the level to proceed forward.
When you have access fast travel, to use it, open up your map and select any of the regions available in the mission. Each region has a selection of checkpoints you can visit, making it easy to jump around to each location.
You can only use this at the end of a level, even if you have already beaten it. If you are going through a level again using the Mission Select function, you still have to complete it and reach the end to unlock fast travel. There is no other way around this, but it makes obtaining all of the collectibles in Doom: Eternal a breeze. Even if you miss one or two of them along the way.
When you repeat a mission using Mission Select you can also use any of the cheat codes you have collected while playing through the game. You can use these to enhance your experience and make killing some of the stronger enemies in Doom: Eternal extremely easy.