Sorry if this is too basic.
In an OLS regression given by
$b$ intercept, $a$ the slope.
Then $a,b$ are not numbers but random variables.
I find this confusing since I start with data points $(x_1,y_1),(x_2,y_2),..(x_n,y_n) ; x_i \neq x_j $ when $i \neq j $. Then we find the line of best fit, which would give us actual numbers $a,b$, so that these seem to be constants, not (Random) variables.
Is the reason these are called variables that I am selecting just one of many possible values $y_j$ for a given variable $x_j$?