These labels are a confusion of physical description and functional description.
You can confound that with the mixed use of the term slipper.
As far as I know, you can rely on the term flip-flop to mean something that is very likely to make a flip-flop noise when you walk in it. It is somewhat loose-fitting and you could kick it off without any bother.
It could be made of anything, but is typically synthetic.
Slippers are generally worn about the home, and only there. They are comfortable and might even be whimsically-designed. Examples abound. They are not intended for outdoor use, or indeed for walking in.
Sandals, on the other hand, or should I say foot, are specifically intended to be worn outside. Sandals are shoes with holes in.
Back to the confusion about slippers.
This may be a local phenomenon, but in addition to the above definition, this term is used here to refer to any lightweight footwear that is not definitely a shoe. So, for example, footwear for sports activities may be referred to as slippers. I think that you can probably forget this paragraph unless you are going to be talking footwear in Northern Ireland and wish to understand the small minority of locals who play tennis, for example, in tennis slippers. But who knows ...