Respawn announced that Apex Legends Mobile will start rolling out for testing in several regions along with the start window and eligible devices in the beginning.
Apex Legends Mobile will start beta testing during Spring 2021, which we're already in, so the actual testing sessions are close to the start. That said, Respawn didn't specify the exact start date.
They did, however, state that the testing will first become available in India and Philipines, which is not unusual - these countries are the leaders when it comes to engagement in mobile gaming. Initially, it will be "just a few thousand" players even in these regions but the number will gradually increase and the test build will become available in the rest of the world.
Furthermore, the initial testing will be available only on Android devices, meaning iOS device users will have to wait for a bit before they get their hands on the mobile version of Apex Legends.
The team is looking to preserve the regular Apex Legends feeling from the PC but keep in mind that some compromises will have to be around due to mobile devices' limitation. Since this is a different version of the game, it will not have crossplay with other versions.
Respawn Entertainment kept the visionaries from the original team in the development circles of Apex Legends Mobile but only in a limited capacity since the game is described as a "collaborative effort with [their] established Apex Legends team and other partners".