How to make platformio knows that where the libraries locate at?

Thanks a lot @manuelbl
It’s amazing. Now I understand the usage of lib_deps.
In the platformio.ini file:

[env:uno]
platform = atmelavr
board = uno
framework = arduino
lib_deps = 
    SD
    IRremote

I find that the libraires introduced by lib_deps automatically located at the following path after compiling:
Snipaste_2020-08-22_14-47-56
What’s the mechanism behind this approach? In the platformio.ini file, I just set

 lib_deps = 
    SD
    IRremote

I didn’t specify a URL or a path, how does the platformio know that where to download the libraries of SD and IRremote?
Best regards.