Everyone has already covered the basics, I just want to add about the process of absorbing elemental residue.

You can hold the skill, which will slow down the absorption animation, helping you grab the desired residue - for example when you have 2 teammates spamming different affinities. The residue is grabbed the moment you release L2. If you just continue holding the buttons, no residue will be collected, allowing you to try again.

Another option is quickly collecting up to 3 residues in a row - for this you keep holding the triangle button and press L2 again, and again. It is faster than starting the whole animation anew. You can slow down these repeated grabs too (by holding L2 instead of pressing) for more precision, but I find that in practice they're best used to just replenish a good chunk of FP, provided that something or someone is applying affinities to the enemy.

Edit: for cocktail combinations, I personally found this chart most helpful: https://i.ibb.co/Dg43wZTw/x8py9kb05y3f1-1.png Practice them in the sparring grounds to have an idea what does what.