In Loki, the Time Variance Authority made use of doorways to traverse both the Multiverse and time itself. It made for an exciting visual but wasn't something we gave much thought to until Earth-838's Mister Fantastic joined his fellow Illuminati members in Doctor Strange in the Multiverse of Madness.
That technology bore a striking resemblance to what we'd seen the TVA use and, more importantly, Doctor Doom's Time Platform from the comic books.
It's factored into a number of stories over the years, including the Fantastic Four's first meeting with Rama-Tut, a Kang Variant who found himself trapped in Ancient Egypt. After the villain planted hieroglyphs he knew would grab the attention of Marvel's First Family (he was aware of them due to the fact he originated from the 31st Century), the team did indeed "borrow" Doom's Time Platform and battled the villainous Rama-Tut in the past.
In Ant-Man and The Wasp: Quantumania's mid-credits scene, Immortus, Rama-Tut, and Scarlet Centurion meet with an entire Council of Kangs, and each of the Variants is shown arriving via a near-identical time platform to the one Reed Richards used in the Doctor Strange sequel.
While Kang has his own means of travelling through time in the comic books - such as the Time Chair we saw him trying to repair in the Quantum Realm - it appears the MCU's take will primarily use technology that, on one Earth at least, was created by Mister Fantastic.
As for how all of this ties together, we need to look at Kang's alleged links to both Reed and Doctor Doom. It's never been fully confirmed either way, but across the years, the time-travelling villain has believed himself to be a descendant of both the hero and villain (which of them is actually his real ancestor is unclear).
With that in mind, Kang may be making use of the technology either created, though we've seen that Earth-838's Reed utilises it as a means of teleportation rather than a way of travelling across realities. Centuries later, however, it's easy to picture Nathaniel Richards taking that and turning it into the device that helped him - and his countless Variants - discover other worlds.
While there have been rumours Deadpool 3 will set the stage for Avengers: The Kang Dynasty, we have to believe Fantastic Four is where Marvel Studios plans to start really tying everything together. Remember, there are rumours the heroes spend years lost in the 1960s before returning to the present day, and that could be with the time-travelling technology which ends up in the hands of Nathaniel and ultimately creates the Kang Variants who bring chaos to the Multiverse.
As for how Doctor Doom could factor into this...well, that remains to be seen, but we'd be remiss to ignore the role he played in the Secret Wars comic book...