Computer Configuration > Administrative Templates > Microsoft Edge
Both
HKLM\Software\Policies\Microsoft\Edge
The 'SitePerProcess' policy can be used to prevent users from opting out of the default behavior of isolating all sites. Note that you can also use the 'IsolateOrigins' (Enable site isolation for specific origins) policy to isolate additional, finer-grained origins. If you enable this policy, users can't opt out of the default behavior where each site runs in its own process. If you disable or don't configure this policy, a user can opt out of site isolation. (For example, by using "Disable site isolation" entry in edge://flags.) Disabling the policy or not configuring the policy doesn't turn off Site Isolation.