Your download should start by itself. If nothing happens, start it again — it's the same disk image either way.
Orbit.dmg · 61.0 MB · SHA-256 e1cac0b36db44983ba1ec900b8f49af8ff15737c03e735e6dca7b8a7cf01653e
Double-click Orbit.dmg in your Downloads folder.
Then open it from Applications. The first time, macOS asks you to confirm an app downloaded from the internet — choose Open.
Orbit walks you to System Settings. Turn on Full Disk Access, or skip it and grant folders one at a time.
Mac App Store edition — coming soon