The thing about 30m vs 40m, I honestly find it confusing too. I have read over and over again that PADI allows AOWs to go to 40m max, but except for the dive shops/guides who push limits a bit or aren't super strict about enforcing max depth, every single dive shop with official max depth has always set it at 30m, not 40m. A very experienced SSI instructor also told me it's 30m.
To further confuse the issue, many agencies offer the Deep 40 (no deco) class and also the Deco 40 class. If the point of Deco 40 is simply to introduce divers to deco and not meant to teach them to go deeper, then what's the point of Deep 40 without deco if PADI already allows AOWs to dive to 40m? And how does it compare with other agencies' AOW-equivalent courses, such as RAID's Explorer 30 and Advanced 35 courses vs PADI's AOW? (I understand the differences between these two courses. The confusion is just about the allowed max depth for PADI's AOWs) And how about Rescue Divers of different agencies. Please correct me if I'm wrong, but aren't there agencies saying RDs can go to 40m? Nowadays nobody allows me to go below 30m without getting Deep certed.
Also before I got my AOW, nobody let me night dive with only my OW.
I understand all these discrepancies can feel very confusing to new divers.