I know that habit of using messenger app for notes :) Try [matrix] using Element or SchildiChat. It’s e2eencrypted and federated, and even supports markdown formatting in messages. Also some Slack features are included. The only issue for non-techy person might be verifying sessions - just be sure to use the same client on all devices.
We used to joke in 2010s: what is Internet Explorer? It’s a program that is used to download a browser.
Nowadays, what is Mozilla Firefox? It’s a repository you fork to make a browser.