thank you for your understanding and patience! welcome to nostr!
Discussion
if I am an old user, and coming back after browser reset or on another browser, I select the bootstrap relays. After sync, are the bootstrap relays overwritten with my old relays? Or a union of old and new is set?
yes bootstrap relays will be overwritten with your previously set relays. bootstrap relays only remain when no relay config can be found for the user (aka new user).