ADB installs apps for all users. How to force it to install to main user only?
Mention the specific user as an argument for which the package is to be installed. Example:
adb install --user UID YOUR_APK
adb shell pm install --user UID YOUR_APK    # alternative command

UID for main/primary user is always 0.
Sidenote: in case of difficulty with an adb command, simply enter adb as the sole command. It would list help section.