Linux has a built-in keyboard that's easy enough to configure in your DE or with an X keyboard toggle.

Dyalog has some introductory documentation on how to set up the mappings. https://www.dyalog.com/apl-font-keyboard.htm

If you want to type something very small, you could also go to tryapl.org and use the on-screen keyboard and copy and paste.

If you want to work with APL, you probably want to install the free APL fonts and keyboard mappings from Dyalog at https://www.dyalog.com/apl-font-keyboard.htm

If you just want to play around with it, you might try https://tryapl.org/

Usually digraphs or special keyboard maps are used.

See https://apl.wiki/Typing_glyphs