What exactly makes a tea "milky" or "creamy"?

I went through the same thing with malty, buttery and more recently savory. Creamy can be a flavor, a mouthfeel, or both. I will leave it to others to give examples. Once you see it, its obvious.

I think that the milky can be from nature or an additive some tea companies will put a dried milk into the tea to create this flavor while others have it in the tea as a natural flavor that comes out.
To me it is both a mouthfeel and flavor either or, or both.
A tea can TASTE milky or creamy but not have that heavy like nature to it and be light or bright in the mouthfeel.

I’m sure others will have better answers but for what it’s worth “milk oolongs” (ones with out any additives) are typically picked in the summer.

Here is a study that seems to have identified one of the compounds responsible in green teas: http://www.aseanfood.info/Articles/11018352.pdf

Many of the components that are responsible for mouthfeel in tea are non-volatile, so separating them from one another for identification and study has been a challenge, it looks like.