Hey there, hkfFL.
Welcome to the Apple Support Communities! Thanks for posting about voice memos not being available across your devices. We'd be glad to help you out with this and provide some information that you might find beneficial.
With the Voice Memos app on your iPhone, saving a voice memo will be saved using iCloud to any compatible device that has the same Apple ID signed in. This would include devices running macOS Big Sur and iOS or iPad OS. The reason you don't see the Voice Memos app on your Mac is that this app was introduced later than macOS High Sierra, which you mention as currently installed on your Mac. If your Mac can run a later version of macOS, you might consider updating if you'd like to take part in this feature. Learn more about the potential of updating your Mac in How to update the software on your Mac. Prior to updating, creating a backup in Time Machine or another backup utility is recommended.
Currently, creating a voice memo on Apple Watch does not sync with other devices like it would from your Mac or your iPhone. If that's a feature you'd like to see added in a future release, be sure to provide Product Feedback here.
Have a great day.