Doom: Eternal is a high octane race that lets you go through all levels of the game. However, the problem is that you can easily miss some of the collectibles hidden throughout the game. To make it easier for you , you have access to a fast travel system to browse the map.
Unfortunately, you can only access this fast travel system after leveling up and reaching the end. At the end, just before completing the mission, a pop-up notification will appear on your HUD informing you that the Fast Travel System is now live and you can use it freely when you pull the map. You can then use it to jump around the different areas, collect any collectibles you might have missed, and then complete the level to continue.
When you have access to fast travel, to use it, open your map and select one of the regions available in the mission. Each region has a selection of checkpoints that you can visit, making it easy for you to navigate to each location.
You can only use it at the end of a level, even if you have already beaten it. If you cross a level again using the Mission Select feature, you still need to complete it and reach the end to unlock a Fast Travel. There's no other way around this, but it does make getting all of the collectibles in Doom: Eternal a breeze. Even if you miss one or two along the way.
When you rehearse a mission using Mission Select, you can also use any of the cheat codes you collected during the game. You can use them to improve your experience and have some of Doom's most powerful enemies killed. : Eternal extremely easy.