What you're seeing is usable Ram, the rest is reserved for the OS and core phone functions. Completely normal, no crime was committed here.
We believe you... some apps report around 10.3 to 10.5GB of Ram, I can see it on my end too... but try this: Go to Settings, Device Care, Memory and in there you will see it says 1.7GB reserved. So that's your 10.3 of user accessible Ram for apps, plus the reserved 1.7GB, a total of 12GB. The reserved portion is to ensure your phone's OS runs fast and smooth and doesn't have to compete with apps if you push it, as well as phone's hardware features are supported all the times, like network features which have to always run in the background to make sure you receive calls, texts, emails, notifications and so on.
I think what might be confusing is the fact that in older models, there was less reserved Ram. My Note10+ shows around 800MB reserved vs 1.7GB on the Note 20 Ultra. My guess is that's because of 5G, and they reserve extra Ram to support these ultra-fast connections. I just can't think of any other reason they would do this.
There is definitely 12GB of actual, physical Ram.