Functional fitness training has different meanings and definitions depending on who you ask.
My definition of functional fitness is when the training has a direct benefit of transfer to a skill or movement that you use in the daily life activities, leisure, sport, or work categories.
In this picture I am carrying in a package of water bottles from my car to the house. This was after carrying in all of my bags of groceries. The exercise equivalent of this activity are all versions of weighted carries.
Weighted carries are a great exercise that work all major muscle groups, core stabilization, grip / forearm strength and cardio.
The direct transfer benefits are many but most obviously are seen in carrying in your groceries from your car to your house.
This is just one example.
Functional fitness is not in my opinion, crazy exercises that have no relevance to real life.
There is nothing relevant or required of me to spend a lot of time doing inverted or handstand exercises as I never have spent anytime upside down doing anything except maybe briefly while scuba diving. But if you are a gymnast or CrossFit athlete then it becomes relevant.
Depending on who you are and what you do, will determine what functional fitness is and its relevancy to your training and life.
Don’t waste your time training on movements that have no relevance to anything in your life, unless you simply enjoy it and want to do it.