The active ones should be visible in the Synthesizer on the Fluid and Zerberus tabs.
The installed but not active ones can be seen either by pressing the Add button on one of those two tabs, or by using your computers File Browser and look in the relevant folders. Which folders those are are listed over at https://musescore.org/en/handbook/3/soundfonts-and-sfz-files
Comments
The active ones should be visible in the Synthesizer on the Fluid and Zerberus tabs.
The installed but not active ones can be seen either by pressing the Add button on one of those two tabs, or by using your computers File Browser and look in the relevant folders. Which folders those are are listed over at https://musescore.org/en/handbook/3/soundfonts-and-sfz-files