Objects... Classes... How do they work?! (LCD library on multiple files)

I’m trying to use the LiquidCrystal library. It has this line:

LiquidCrystal_I2C lcd(0x23, 2, 1, 0, 4, 5, 6, 7, 3, POSITIVE);  // Set the LCD I2C address

I have multiple files: file1.h, file1.cpp, file2.h, file2.cpp, header.h (general header), main.ino.
Where should I put that line, and where do I need to #include it’s libraries

#include <LiquidCrystal_I2C.h>
#include <Wire.h>

if I want to use the functions it provides (lcd.print(), lcd.setCursor() etc.) in 2 ore more .cpp files?

Thank you very much.

Where should I put that line, and where do I need to #include it's libraries

In the main .ino file. That creates a global variable.

In other files, where you need to use the global instance, you need to include a statement that says that the instance is extern to the file.

So put this:

extern LiquidCrystal_I2C lcd;

in the files that I need it?

(sorry, not sure what an "instance" is)

(sorry, not sure what an "instance" is)

A class is a cookie cutter. An instance is a cookie. The cookie cutter (class definition) was used to create a cookie (an instance of the class).

So put this...in the files that I need it?

Yes.