To an outsider, "Buddhism" appears to be (more or less) one monolithic tradition. You sit, you meditate, you attain nirvana. Maybe practitioners in different countries (or even adjacent monasteries in the same country) wear different colored clothes or hats, but that's basically it.
After years in search of what constitutes Buddhist practice, I find I'm far less sure than when I started. No doubt many (probably nearly all) practitioners will vehemently disagree with what I'm about to write below, but there seems to be mounting evidence that the sub-schools heartily disagree with each other, and none seems to be truly "authentic."
I've done no original research here; I'm just sharing some resources and what I've learned from them. We'll focus on three rough schools of Buddhism: Theravada, Zen, and Tibetan Buddhism.
1. There is no agreed upon definition of jhana, or technique to attain it
This paper explains how the details of the most fundamental meditative states emphasized by the Buddha (the jhanas) are not remotely agreed upon even amongst Theravadins. To what level must they be developed? Are they necessary for enlightenment? No two (sub-sub-)traditions seem to agree.
2. There is no one agreed upon meditation technique even within Theravada
In the above post, the author describes how Theravada (the school most often considered to be aligned with the words of the historical Buddha) actually has no lineages of practice that can be traced back before about 1900.
Vipassana was reinvented by four people in the late 1800s and early 1900s. They started with descriptions of meditation in scripture. Those were vague and contradictory, so the inventors tried out different things that seemed like they might be what the texts were talking about, to see if they worked. They each came up with different methods.
Since then, extensive innovation in Theravada meditation has continued. Advocates of different methods disagree, often harshly, about which is correct.
3. The (one) man who brought Buddhism from India to China, which later spread to Japan, which became "Zen", may never even have existed
Several scholars have suggested that the composed image of Bodhidharma depended on the combination of supposed historical information on various historical figures over several centuries. Bodhidharma as a historical person may even never have actually existed.
4. The "pinnacle" of Tibetan Buddhism, Dzogchen, was introduced some time after 55 CE, by a man born in modern-day Pakistan, who it is freely admitted has no (earthly) connection to the historical Buddha
It is held that he received the teaching from a celestial Buddha, of whom the historical Buddha was a manifestation. Not only that, but the same teaching seems to have existed in the aboriginal religion of Tibet. They claim that it was first taught by a man who predates the Buddha.
5. No two traditions seem to agree on the endpoint of practice
In Theravada, it is traditionally held that one practices to become an arahant. In Zen, one aims to become a Buddha via the bodhisattva path. Meanwhile in Dzogchen, they claim that only Dzogchen practitioners attain the ultimate realization called the rainbow body.