photos on external hd or stored as entire photos library

Just open the old library.

Hold down the option key when starting photos. If the library you want to open is listed, open it. If not use the choose library button.

To go back to your normal library, you do the same thing - option key open.

You can also simply switch between libraries from the Finder by quitting Photos and then double clicking the library you want to browse. This will work, as long as the external volume, where the library is stored, is compatible with Photos. You must not open a library while it is on an incompatible volume anyway, as the library might get damaged: If the library is in an unsafe location move it to a compatible volume: Move your Photos library to save space on your Mac - Apple Support