Just like Windows 10 19H2/1909, the Windows 10 20H2 update will be a quick install for anyone running the Windows 10 May 2020 Update/2004. (Anyone on a version of Windows 10 earlier than the May 2020 Update will see the coming 20H2 release as a regular, larger update.)
The new test build out today is available to Insiders in the Beta Channel who "seek" it out. That means to get it, Insiders need to go to Settings > Update & Security > Windows Update and choose download and install 20H2. Once Insiders are on this build, they will get regular 20H2 updates via Windows Update. Those in the new Beta Channel who opt not to download and install 20H2 won't get new features.
Microsoft is using this new approach to get the test bits to Insiders rather than going back to the approach it used to test 19H2. With 19H2, Microsoft released bits to some Insiders with features turned off and to others with features turned on. That strategy frustrated and confused some testers. Instead, Microsoft will be providing Insiders with the 20H2 build installed with new features as they are delivered.