The thing is, you and your Plan Committee (a group of faculty that work to help you define your Plan) work together to decide how best your classes can fulfill your plan.
If you study film and you’re interested in making nature documentaries, then you’ll probably be taking a fair bit of natural sciences and a fair bit of video. If, however, you want to go to grad school for biology, the game changes - your Plan Committe will help you pursue the question of “what part(s) of biology am I most interested in?” as well as helping you to know what kinds of courses will be in a typical pre-req requirement for a Master’s program.
It’s apples and oranges, basically. Don’t think of the Plan as ‘building your own major,’ necessarily (though there is certainly an element of that in the whole process) - think of it as defining an area of inquiry that you want to use your education at Bennington to explore. If your inquiry is about research-level, graduate biology, then you’ll have to conduct summarily high-level classwork in order to graduate. If you’re asking questions about the natural world via film, then your classwork will likely be spread out more between the two disciplines.
But yeah! I hope that makes sense. Definitely read up on the Plan Process in that link in the first paragraph, I think everything will make a lot more sense afterwards. I hope this was helpful!