The original Doom games, Doom and Doom 2 were great shooters of their time. In fact, these were the games that gave us a taste of what it is like to play an action-packed first-person shooter. What made the Doom games some of the most enduring games of all time was the support for external WADs, which allowed players to create new levels and share them with the community. Bethesda has now revisited those classic titles and brought back the same sharing functionality and support for "add-ons." Bethesda describes these add-ons as a curated list of content for Doom and Doom 2, as well as "some of the best community episodes and megawads," which can be accessed from both the games' main menus.
The add-ons are going to include The Plutonia Experiment and TNT: Evilution. Apart from that, Bethesda will also add professional and fan-made content, which were drawn from the 25-year-old history of Doom and Doom 2. Moreover, all of this is going to be absolutely free. Bethesda also stated that it has "already begun tracking down members of the Doom community to discuss releasing their add-ons, and hope to release many more in the future."
Both Doom and Doom 2 have been enhanced to run at smooth 60fps instead of their original 35fps. There are also various quality-of-life updates made to the games, including easier level selection and reliable aspect ratio management.
The enhanced Doom and Doom 2 are separate products. However, if you already own the original Doom and Doom 2, Bethesda will add the Doom Classic (2019 release) and Doom 2 (2019 release) to your game library. Also, if you purchase any of the two versions of Doom and Doom 2 in the future, you will get the other versions as well.
To read the entire rundown of changes in Doom Classic and Doom 2, you can head over to bethesda.net and read the patch notes.