[PlatformIO] Where is Serial defined?

I'm making the transition from the official IDE to PlatformIO CLI, and I've run into a problem. While including Arduino.h gives me access to functions such as millis() and digitalWrite(), I can't use the Serial methods. I know that HardwareSerial.h is the header I need, and I know where to find it on my machine, but I find it suspicious that I can't find it in the PlatformIO's repository.

Anyways, the command

pio lib --storage-dir /opt/arduino-1.8.15/hardware/arduino/avr/cores/arduino/ list

doesn't find anything, yet that's where the headers are. I must be doing something wrong.

My path is
Arduino\hardware\arduino\avr\cores\arduino\HardwareSerial*

There may be more than the avr\ dedicated directory, and more related files in other directories.