Marvel's ever since Avengers: Endgame and Loki, the MCU has dived into exactly how time travel works - and so has the comics.
The events of Avengers: Endgame depict Earth's Mightiest Heroes traveling back in time to retrieve the six Infinity Stones to save their own present. While the film stayed within a singular timeline, Loki reveals that multiple timelines exist - or did exist, until the establishment of the so-called Sacred Timeline. By the end of the series, that Sacred Timeline has been invaded by multiple timelines and the guardian He Who Remains has been killed, and the Marvel Multiverse truly begins.
In the Marvel Comics universe, traveling to the past and changing a key event didn't actually change the timeline - it merely opens up a branching path through time and creates a divergent timeline. This changes with the Kang-centric series Timeless, in which a rogue timeline is reattaching itself to the "prime" timeline, perhaps better known as the Sacred Timeline from Loki lore. Other rules are similarly changed: while realities are still designated Earth-x, timelines can be designated too (20208 Manticore Green, for example).
The concept of "Variants" (also from Loki) has also been introduced to the Marvel Comics universe, as seen in the 60th Fantastic Four Anniversary issue in which various Kangs are welcomed as such. The TVA's explanation of the Reckoning War, featured in the Fantastic Four books, states that the conflict is a fixed event in time - but wouldn't that be the case for all events in the Marvel Universe if the old rules concerning the inability to change events still applied? Note that the TVA officers say "The moon's [destruction] is new," implying that variables can and will change within those set-in-stone events.
It's no secret that Marvel, realizing the films are an easily-accessible gateway to the comics, has decided to synchronize the separate universes as best as possible. This involves changing certain characters' looks, bringing back certain groups such as the Eternals for a new 2021 run, and elevating Kang in the comics in anticipation of the Kang from the films. Kang the Conqueror has changed both the Marvel Comics universe and the MCU irrevocably, merely through his presence alone.
