Much of this availability has to do with the form of the element. To be useful to the plant, the elements generally have to be in an ionic form, which is why some fertilizers chelate their metals. Nice healthy soils with lots of organic matter or clay essentially behave like ion exchange resins, holding on to and stabilizing the ionic species for eventual use by the plant. If the pH is too high, insoluble metal oxides (Cu, Fe, Mn) can form. If it’s too low, too much Al3+ is released from rocks, which displaces all the other elements on the ion exchange sites.
Redox properties can also have an effect. Well aerated and well drained soils tend to be oxidizing while wet, decomposing, clay soils tend to be reducing. In the presence of sulfur, anaerobic conditions lead to formation of insoluble metal sulfides. Anaerobic conditions also increase Fe availability by changing its oxidation state from Fe3+ to Fe2+ (which I assume makes it easier for its release from the ion absorption sites. There is a lot of complicated chemistry going on here.
I admit that I was totally unaware of any of this when I started gardening. In fact, the only reason I even know this much is due to having to understanding how to change soil pH. Lime is used to raise soil pH while iron salts and elemental sulfur are used to lower soil pH. The lime and iron reactions I understood from freshman chemistry, but sulfur? Gardeners would simply tell me that sulfur is acidic, but that obviously wasn't true. Eventually I guessed (correctly) that the sulfur oxidizes to SO2 which dissolves in water to make H2SO3.
What do I do with all this knowledge? Well, nothing really. I just plant the flowers, add Miracle-Gro, and hope everything grows.