Just bought the Samsung galaxy s5e tab with Android 9 installed I have turned on the developer option but I cannot find
where to turn on the option to use apps from unknown sources.
Could be wrong but you authorise the location of the .APK file now.
If it's stored on the device you authorise the file manager to install from the unknown source. Good idea really as you can allow .APKs from storage but that doesn't give blanket authorisation to all sources eg browsers and other apps.
Settings/bio and security/install unknown then choose the source.
Choose the source.. There's a star wars joke in the somewhere.