Setup guide
About two minutes, once. The app walks you through all of this and ticks each step off by itself. This page is the same thing in writing, for reading ahead or helping somebody else.
macOS will not let an app provide a file system until you say so. In System Settings, open General, then Login Items & Extensions. Scroll to the bottom, to Extensions.
Switch that list to By Category. This matters: under By App the File System Extensions row does not exist, which is where most people get stuck. Open File System Extensions and turn SideMount on.
The button in the app opens that exact pane for you.
Android hides this behind a small ritual. On the phone:
The wording moves around between makers. On Samsung, Developer options sits at the bottom of the main Settings list. On Xiaomi, tap OS version (MIUI version on older phones) rather than Build number. The USB debugging guide has the steps for each maker.
The phone asks whether to trust this computer. Tap Allow, and tick Always allow from this computer so it stops asking.
If no prompt appears, unlock the phone first. Android will not show it on a locked screen.
The phone appears in the SideMount panel with a Mount button, or mounts by itself if you left that setting on. Open it in Finder from there.
When something is wrong
It is the cable, four times out of five. A cable that came with a power adapter often has no data lines. Try the one that came with the phone, and a different port.
Unlock the phone and look for the prompt. If you have dismissed it before, go to Developer options and tap Revoke USB debugging authorizations, then unplug and plug in again.
macOS runs the file system service, and it can get into a state where nothing will mount, including Apple's own file systems. Apple does not allow that service to be restarted, so the cure is a restart of the Mac. Ejecting normally rather than yanking the cable is what avoids it.
A phone offers one connection at a time. If you have Android Studio or an adb server running, quit it and press Reconnect in SideMount's settings.
More causes, and the fixes for each, are in Android phone not showing up on a Mac. Still stuck? Email us and say what the app shows you. It is a small enough operation that you will get a real answer.