Canon

Canon would refer to the body of works related to a particular subject and considered proper history and continuity to series.

Background
In case of Doom there are very few official comments that relate to canon or discuss canon directly. Sometimes official positions are contradictory or changes over time (or retcone are made).

As such there are a number of fan theories as to what comprises canon or what is not canon these can only be considered Fanon as their is very little as far as confirmation on what is canon.

In addition the Doom series has received a number of spinoffs and reboots throughout its life as well as tie ins.

Each reboot could be considered separate canons or alternate universes as part of a multiverse (Quake Arena backstory for example, Arena Eternal outside time and space, with heroes from various “alternate universes”).

Tom Hall and John Romero have confirmed that several ID series are in fact connected. Doom Marine is a Blazkowicz descended from Commander Keen, and William J. Blazkowicz before him (this does not apply to the MachineGames reboot universe however which follows its own universe with an alternate history for B.J. And different family tree). This is also mentioned in Doom RPG series as well. See ID Universe timeline.

In both Doom RPG and Doom 3 boxset the hero for most of the games is alluded to be the same hero in Doom 1-3 (and the RPG series).

Quake Champions brings back Doom (Quake III Arena), now listed as Doomguy and calls him Doom Slayer and mentions his pet Daisy confirming a link between Doom 2016 and Doom 1. The game also brings new Machin Games William Blazkowicz from his “universe”.

The Fanon tends to argue against the veracity of some of these references https://www.resetera.com/threads/keen-blazkowicz-and-doomguy-are-all-related-tom-hall-confirms.19853/, assigning different levels of theorized canonicity to the various references. But for most part ID’s internal position is not known.

Crossovers

 * Wolfenstein, Commander Keen, and Doom are connected according to Tom Hall.
 * Wolfenstein RPG and Doom RPGs include crossover material with a number of ID series and each other.
 * There are some crossover material between Doom, Doom 3, Doom (2016) and Quake Arena III, Quake Champions. The latter also has crossover with MachineGames Wolfensrein universe.
 * A comic in Quake for Saturn included and adventure following Quake Marines Dank and Scud run into the Doom Marine during their adventures.