User menu: How do I define the liquidCrystal in my custom library? (1602)

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Your question is: How can I use class/object A in my own new class/object B.
Unfortunately, the answer is: There are two options. You have to decide which one to use:
Composition or Inheritance

Both methods have thire pros and cons. It is difficult to explain without going much deeper in the technical implementation. So, let me just answer your question like this (now knowing that the actual answer is much more complicated): Use composition, which means, that you define a member variable, which is a pointer to the LiquidCrystal object:

class myclass
{
 LiquidCrystal *lcd;
}

You can define a member function, which must be called by the user to set the LiquidCrystal object:

myclass:setLC(LiquidCrystal *_lcd) { lcd = _lcd; }

Of course you can use the existing member function of LiquidCrystal in your member functions:

void Menu::printMenu(){
	lcd->clear();
	lcd->setCursor(0, 0);
	Serial.println(_lB.length());

Better answers might be in the programming section of this forum.

Oliver