The GPL makes it a pretty clear distinction, "preferred form for modification" is pretty clear, but decided on a case-by-case basis. Obfuscated code is not source, generated code is not source, transpilation is often not source but could be depending on how you use it afterwards, bitmap images are often not source but they can be, executables are usually not source but could be, videos are not source but could be. Some links discussing what source is here:

https://www.inventati.org/frx/essays/softfrdm/whatissource.h... https://b.mtjm.eu/source-code-data-fonts-free-distros.html https://wiki.freedesktop.org/www/Games/Upstream/#source https://compliance.guide/pristine https://opengameart.org/forumtopic/source-required-for-art-l... https://wiki.debian.org/rly-free-software