Infinity is not the expression of a number, but the expression of a limit.
It is neither. But a function or sequence may have infinity as its limit somewhere.
Although the value of a number or a function cannot be infinite.
Of course a function can be infinite, just not a function that has the real numbers as its range, which is probably what you are used to.
This means that, sometimes, it will be possible for us to use infinity as if it were just another value, and perform operations with it.
I think i see where you got this from. You are talking about functions within a limit expression. When we see a term like
lim(x->0) (g(x)/f(x)) or anything like that, we may say g goes to infinity or f goes to infinity with x -> 0, however it is important to note that when you are performing any arithmetics on these functions, you are not dealing with infinities. The limit is applied at the end, you are just performing arithmetics on normal numbers or functions.