This page of our guide to Wolfenstein: Youngblood explains where to find an easter egg that lets you play the classic Wolfenstein 3D installment of the series. It's a nice accent for fans of older first person shooters.
You can play Wolfenstein 3D when you reach the Catacombs, a hideout of the local resistance. Catacombs are visited for the first time after completing the second mission (Riverside), but you can also return to them as many times as you like.
Enter the bar in the hideout area, where there are two slot machines. Interact with any of them.
The game will zoom in on the image - the edges of the screen will still show fragments of the machine. Wolfenstein 3D game is fully playable. You can select the episode you are interested in, as well as preview the controls and set the difficulty level. Wolfenstein 3D allows you to save games, so you don't have to complete a game in a single run.