What is whey when making homemade yogurt?

When I create yogurt and drain the whey to make greek yogurt I started thinking what is whey? Does it still have protein or is it just slightly acidic water?

Yogurt Whey has protein. Some proteins in milk coagulate in yogurt production, and will stay with the solids when strained. Some remain in solution, so they drain away with the whey.

Yogurt contains whey and casein in roughly the same proportions as those in original raw milk, although some of the proteins are partially broken down by fermentation [57]. Approximately 90–99% of the whey protein in raw milk is denatured by heat treatment prior to fermentation (generally 5–10 min at 90–95 °C) [58], and denatured whey proteins, such as β-Lg, are combined with casein micelle [59]. In addition, the decrease in pH due to lactic acid fermentation causes the casein to approach its isoelectric point (approximately pH 4.5–4.2), resulting in a decreased electrical repulsive force among casein micelles (Figure 1). Because the pH-decreasing process in fermentation is gradual and slow, the casein micelles in yogurt do not coagulate and precipitate as hard as in the case of abrupt contact between raw milk and gastric acids in the stomach. In the yogurt fermentation process, the casein micelle particles are hydrated and form a homogeneous mesh-like structure, which becomes soft white tissue [13] (Figure 1). Because whey proteins that do not combine with casein micelles are soluble in this gradually decreasing pH range, they remain dissolved and present in trapped water in the above casein micellar network or in the released free water from the yogurt structure.

Quote Source: https://pmc.ncbi.nlm.nih.gov/articles/PMC10609537/

There are also minerals (as the first-linked Wikipedia article mentions) and for instance I find checking the potassium content revealing of whether a "Greek" yogurt in the USA is actually strained, or merely hit with a bunch of thickeners (which some less reputable brands do, as apparently the term is not defined by regulatons.) For the same serving size, a strained yogurt has less potassium than unstrained, whether the unstrained is "normal yogurt" or "Allegedly Greek by way of e.g. corn starch and pectin" (apologies to the rest of the planet for our horrors...)