Letters from the Greek alphabet are used in physics, mathematics, chemistry, etc. However, I cannot include them in my questions as they aren't available on the standard Android keyboard.
I am assuming that you mean how can you include them in answers on SE, in the markdown editor. You can just use the HTML equivalent, so for example if you are on SE.Electronics, and you need the unit of resistance, Ω, just type
You can get the upper and lower case letters by changing the case of the first letter in the name, i.e for ω, type
You can find the respective HTML greek character codes on UTF-8 Greek and Coptic, but is quite intuitive so you shouldn't need to look each one up.
and so on.
All of the characters used in these HTML codes are, or should be, available on your Android keyboard.