First I have to say that I like your title wording.
As for your issue. You mention your memory storage which is different than RAM. The two are not really related in any way except that if something was writing to your memory it might require some CPU to do that which would lower your RAM during the process.
B. Diddy's app suggestion may help you identify your storage issue but Samsung actually has a pretty good tool built in to it with Device Health. I don't believe they started calling it Device Care until the S10 series in 2019 however they are both the same.
In Device Health/Care you should be able to see both your Memory/CPU usage and your Storage usage.
Opening the Storage tab should give you a good idea of what is being saved on your device along with some options for helping reduce and manage these items. It will quickly help you identify why you are down to .5GB of storage.
Opening your Memory tab will help you see what is running on your CPU. Optimizing will close older apps that are just being held in RAM and do other things to bring your usage down to around 50%. It's normal for a device to hover around 50% when idle so don't be alarmed if you don't see this fall below half. Ultra high or low CPU usage that remains that way would be cause for concern. Obviously CPU usage will go up and may near capacity when gaming or using a highly interactive app. If it dips too low your phone will seem sluggish. Apps may take longer than normal to open or systems may crash.
Since you've already restarted your phone there may not be much to see here as restarts typically trigger many of the optimizing actions.
If that notification returns however you need to tap and hold it. This will reveal what app sent the message. If it is from Device Health/Care it is likely a legitimate warning and may require action on your part. It would also likely tell you which app is responsible which is useful and can help you decide your course of action. However I feel like this was a nefarious notification from a bad app. I would also risk a guess that had you simply clicked on it you would have been taken to an app store and offered some kind of protection app. Ads like these are not uncommon and are meant to scare the user into taking action, especially installing an app that claims to fix, solve or protect the user from whatever it is, i.e. viruses, malware, storage shortages and in this case a slow device due to CPU usage.
Visit your Device Health/Care app under settings every once in a while so that you'll have an idea of what is normal for you and your device. Be wary and cautious of anything that warns of trouble if you're not having trouble. Above all don't install anything suggested in an ad without thoroughly vetting it by reading reviews and checking its ratings especially if it first tries to put fear in you by telling you something terrible is happening.
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