Ben, there is a lot of material in the mishnah that definitely predated rebbe. What rebbe did was:
Get together all of the chachamim in his generation and collect each one’s mesorah(see bartenuras hakdoma)
Took the words that had been recited and memorized and edit them together; cutting and pasting essentially.
Paskened according to one shitah and made that shitah a stam mishnah or “chachamim omrim” (see mesechta beitzah 2b)
Ordered them by topic and sub topic (seder and mesechta, perakim etc)
Decide which material should be excluded from the core and left to braysos
Fix up any mishnayos that may have had errors
There’s a machlokes if rebbe wrote down the mishnayos physically or just did the above and make it the standard for klal yisroel going forward