you can see everything (spoilers): in the first half of the movie, it's obvious that the title refers to the always-on camera setup. at this point the title is still tied to the question of elizabeth's motives.
in the second half the title shifts to the making of the movie itself. the most direct example is revealing the discussion of billy getting a cut of the profit. but also, at one point we see nathan showing amanda the first half of the documentary already edited. we watch reenactments being put together, small details in the scripts getting debated and changed, and the motivations of nathan and amanda being considered. the final scene directly questions whether nathan has been biased in how he retells the story. it's clear a lot of thought went into how this could be portrayed accurately (though this becomes a secondary concern as the focus shifts to meta commentary), and "you can see everything" can be interpreted as a justification for that whole process. (e.g. "look, we've shown you how we made this movie, therefore we tried our best to be accurate")
but the point is that we inherently can't see everything, and maybe we can't even know anything. ("i don't know if i know" ironically aptly describes my viewing experience). when the final text appears ("they're not playing me. they're playing a character i created"), it's about themselves too, not just elizabeth.