You are necro-posting in a 2 year old topic. The information here is outdated.
Library IDs are the internal IDs within the PlatformIO library registry, looking into the the libraries files in your local Arduino IDE installation will not help. You need to find the library in the registry. Clicking on one lib will then show in the URL the ID, e.g. https://platformio.org/lib/show/64/ArduinoJson, ID 64 is ArduinoJSON.
However, you should not be using library IDs in the current day and age. You should always be following the official documentation, e.g. regarding the library manager. There it will tell you that you refer to a library, which is in the PlatformIO registry system, with their uploader username, library name and version (as SemVer expression).
PlatformIO again shows you this on every library install page, e.g. with the above referenced ArduinoJson
So you should be using
lib_deps =
bblanchon/ArduinoJson @ ^6.18.3
in the platformio.ini (see lib_deps) instead of the outdated way
lib_deps =
64