I have found WhatsApp useful in a very limited scenario - calling contacts based overseas without incurring international calling fees. Certainly, other solutions exist. One general rule of thumb has that if the service is free, your data is the likely product. Installation of "free" mobile apps willy nilly seems ill advised for technology novices. Likewise advanced practices like rooting a device. 

A senior housing specialist company has a starting overview of online security for seniors.  It's free so they likely want customers for their consulting business. However, the relevant information on the web page can provide ideas for less commercial training material. 

2025 Cybersecurity Guide for Seniors
https://floridaseniorconsulting.com/cybersecurity-guide-for-seniors-2025/#:~:text=This%20updated%202025%20cybersecurity%20guide%20provides%20essential%20tips,users%2C%20making%20them%20a%20primary%20target%20for%20cybercriminals.