Is lower case the most basic calligraphy style? Do I have to use lowercase letters to copy classics?

Strictly speaking, lowercase letters generally refer to lowercase letters, and of course they also refer to uppercase letters or official books. You don't have to use block letters to copy the scriptures, you can use block letters, middle letters, running scripts, cursive scripts, official scripts, seal scripts and so on. The reason why most of the classics are copied in lowercase letters is entirely for practical purposes. Scriptures are for reading, generally speaking, there are many words, so it is more practical and appropriate to copy them in small font.