I believe it depends on the era and system, but there were various APL codepages (i.e. definitions for the upper 128 characters) for both EBCDIC and ASCII.
https://en.wikipedia.org/wiki/Digital_encoding_of_APL_symbol...
In the very earliest IBM Selectric teletype-based systems, some APL symbols were constructed by entering one character, hitting backspace, and overstriking a second character. For instance, ⍋ is | overstruck on ∆. It's why a lot of APL symbols look like that.