so the idea of them from what i heard is that if a man fell in battle that person would be replaced but isnt that how normal infantry ranks worked where if a guy died another would stand where he was, i dont know why they are so well known for this unless im missing somthing
No, that's a corruption of what Herodotus actually says:
ἐκαλέοντο δὲ ἀθάνατοι οἱ Πέρσαι οὗτοι ἐπὶ τοῦδε: εἴ τις αὐτῶν ἐξέλιπε τὸν ἀριθμὸν ἢ θανάτῳ βιηθεὶς ἢ νούσῳ, ἄλλος ἀνὴρ ἀραίρητο, καὶ ἐγίνοντο οὐδαμὰ οὔτε πλεῦνες μυρίων οὔτε ἐλάσσονες
And these Persians were called Immortals on account of this: if anyone of them left their number, either forced by death or sickness, another man was picked and they were never more or less than ten thousand