“What were the “original” tefillos? The way the avos davened?”
The Avos had no nusach, they just prayed from the heart. That’s how it was until the beginning of the 2nd Bayis. Anshei Knesses Hagedolah laid out a basic framework for how a tefillah should be structured. They prescribed 18 berachos, specifying how they should begin and end, and what their general topic should be. But it was still up to the individual shliach tzibur to improvise within that structure. Not everyone could do it, so you went to shul and listened to the shliach tzibur’s prayers and fulfilled your obligation. Chazal said “do not make your prayer fixed”. Only after the gemara’s time, in the time of the ge’onim, did prayer-leaders start writing fixed siddurim. And once that happened everyone who could afford a siddur could daven for himself.
There were several siddurim of the geonic period, but <i>all</i> of our modern nuschaos are based on Siddur Rav Amram Gaon. We don’t know what this originally looked like, because there are many manuscripts claiming to be this siddur, that all differ from each other in details, but the basic outline is the same.