Free will is a necessary assumption (not a matter of observation); a metaphysical thing, coming prior to experience, framing experience.
It is not possible to observe free will nor to measure it; indeed experience has no relevance to the existence, nonexistence, strength or weakness of free will.
Free will is not empirical at all.
Free will is a necessary and inbuilt assumption in (at least!) all creatures capable of recognising the question of free will.
Christianity (and other things) are built-around this metaphysical assumption of free will.
The point, therefore, about free will is not to understand it - free will cannot be understood more than to acknowledge that it cannot coherently be denied - but to use it well.