Thanks for the continued advice and constructive criticism.
Yes, I know how a circuit works. No, I don't need help installing this fixture.
Just had a question about the funky wiring scheme. I've not run across four neutrals of dissimilar metals all wire-nutted together to a light fixture before. That's all. Was stupid of me not to check for juice in these neutrals or turn off the main breaker, (since the neutrals were from separate circuits).
Like the dog who was hit by the car and lived to bark another day, I'll not make that mistake again.