Look at the Pricing page at the Memento Web site...
https://mementodatabase.com.
Basically, there's the Mobile Edition and the Desktop Edition. The Mobile Edition runs only on Android. The Desktop Edition can run on Linux, Mac, & Windows. The editions are priced separately.
The Mobile Edition is basically free, but you can buy the Pro version for a one-time fee of $15, which I gather you did. That gets you a Pro mobile app, not a cloud, other than the Free version, which is very limited.
Also, the Desktop Edition comes integrated with the Memento Cloud, and depending mainly on how much space you want to pay for, it'll be free, $3, $6, or $10 per month. The first two come with the Lite version of the app, and the other two come with the Full version. Unfortunately, the names of these versions overlap with those of the Mobile Edition; specifically, the $6/month version is called the Pro version. Oh, well, just compartmentalize if you can, instead of being confused about that.
I can't say what is different between the Lite and Full versions of the Desktop Edition, but apparently, sync with gSheets is one difference. So, if you want to do that, you'll need to either shell out at least $6/month for the Desktop Edition or do the syncing from your Android device.