I just found this training film from 1943 that discusses the use of signal mirrors. The information in this film is just as true today as it was back then. We sell the true glass mil-spec (MIL-M-18371E Mark 3) Emergency Signalling Mirrors here on our site: https://5col.com/products/mirror-emergency-signalling-mark-3
Low tech skills will rarely fail you. I wonder how many people have half a dozen different nav apps on their phone, watch, etc. but don't have a clue about how to use a simple signal mirror....