Like for example, it's common to use the Greek letter $\theta$ to represent an angle right? So what would a Greek person doing math use to represent an angle? Would they also use $\theta$? Or is there another notation that they would use in order for them to use their letters like we do? Such as if we say $A\geq B$, would a Greek student, mathematician, or whoever say: $\alpha \geq \beta$ or is there something else they say? It just seems like the Greek letters from a non-Greek point of view have so much meaning to us, but then how do they percieve their letters used in mathematics?
The Greeks seems to use the Latin letters together with Greek letters as the rest of us. Here is a screen dump from some notes on Functional analysis. Of course this is just an example.
In my experience with Greeks, they set $a = \alpha$ and $b=\beta$ etc... however, the Greeks I knew were beyond associating a concept with a letter, so perhaps these are not the Greeks which you seek.
I do recall many conversations of the form: "is it "a" or is it "$\alpha$"" to which I would inevitably get the annoyed retort: "yes".
They also use the same (Greek) letters. For example, for angles, it is very common to use $\phi$ or $\theta$ and in equations, they use $\chi$ and $\psi$. Similarly, they use the Greek alphabet (capital letters) Α, Β, $\Gamma$, $\Delta$, etc., for points in Geometry, etc cetera.