Please manually download the 3.11.0 release of Realm Studio from Release 3.11.0 · realm/realm-studio · GitHub this has the ability to open Realms using the new format, but please be aware that opening Realms with this version of Studio will also irreversibly upgrade “old” Realm files, making them unable to open via “older” SDKs.
There’s a feature in macOS called “Gatekeeper” that gives you a level of security against opening apps from unsigned developers. By default it just won’t open them.
However, to provide flexibility, right-click open circumvents Gatekeeper and allows you to open unsigned apps.