I had the same question as the OP of What is the story or reason of how 龙 came from 龍? and I like visuals given by Stan. The hardest part for me in Chinese is remembering the strokes of these simplified characters that came from traditional characters that don't seem very alike. Is there a website/book that explicitly deals with this?
Traditional Chinese refers to Chinese characters that does not contain newly created characters or character substitutions performed after 1946. After new China was founded in 1949, entered a new phase in simplified Chinese characters, simplified characters adapted to early literacy movement. So, Chinese simplified largely promoted the rapid growth of the national. But the policy problem, Taiwan still use traditional Chinese. How to simplified characters came to be? I can recommend the book.
You can find more study cards in https://www.pinterest.com/studychinese/learn-chinese-characters-with-hanbridge-mandarin/
There is no specific rules about how they became simplified Chinese, there is a group of expert did that a few decades ago, the goal is to make it easier to write, and don't lose it's original shape a lot. There is actually even a easier version than the current simplified Chinese, but it lost the spirit of Chinese character, so they were abandoned.