Starting October 20, Windows users who are running "select devices" with Windows 10 1903 or later can proactively seek it out by going to Settings > Update & Security > Windows Update and checking for updates. Once the update appears, users can select Download and Install.
For those running Windows 10 2004, the 20H2 update will be minor and quick to install. That because the 20H2 update is meant to be a small update to 2004, activated with an "Enablement Package," which looks and feels a lot like a Cumulative Update. For anyone running a version older than 2004, the 20H2 update will feel and update like a regular Windows 10 feature update. (This is the same way Microsoft rolled out Windows 10 1909, which was a minor update to 1903.)
As Microsoft has been doing lately with Windows 10 feature updates, it will be throttling the delivery of the 20H2 update, delivering it first to devices where it is likely to have fewer issues. As has been the case recently, Microsoft will likely issue "safeguard holds," designed to prevent the feature update from installing on devices where there are known issues and incompatibilities.
Windows Server Update Services (WSUS) starting today, October 20
Windows Update for Business starting today, October 20
Volume Licensing Service Center starting today, October 20 (though it may take a day for downloads to be fully available in the VLSC)
Visual Studio Subscriptions starting today, October 20
Software Download Center (via Update Assistant or the Media Creation Tool)
There's also a new version of Windows Server rolling out simultaneously with Windows 10 20H2 which is available starting today. Windows Server 20H2 is a semi-annual channel release focused on reliability, performance and general improvements, Microsoft says. It's available starting today on Azure or via the VLSC.
Neither the client or server 20H2 releases are considered Long-Term Servicing Channel releases. The next LTSC release of Windows 10 Enterprise is expected toward the end of 2021. The next LTSC release of Server is in testing now. I've heard from my contacts that the next Windows 10 client and server releases are now being developed independently and may not necessarily arrive at the same time.