This question contains spoilers…
(view spoiler)[In ADSOM, Kell says Antari magic was used to create the doors between worlds. He also says it needs two things to work, blood and a token. How were the first doors created since they would need a door to get a token? (hide spoiler)]
The first doors between the worlds were natural things, gates, much like the Sources of magic are natural, a part of the fundamental design. But over time, as magic settled into shape, the gates became harder to pass through, and the navigation was designed and regulated by powerful Antari, the kind that no longer exist.