In addition to the criterion channel, already mentioned, try MUBI [1]. It’s a cinephile’s dream: a film discovery nearly everyday while also curating the great directors.
One can also find very interesting stuff on YouTube, depending on how good your search skills are, I know that at the start of the pandemic I had discovered a user who was uploading Italian western spaghetti movies in HD format. I think I might also have found something similar for Hong Kong wuxia movies from the 1960s and 1970s but I'm not sure.