If by "legit" you mean "partly hackd"(event unlock etc.) or "glitched" (coincase etc.) then by ninty's standarts it isnt legit (even tho I and many others do think its legitimate...) so...
If you got it on gb/c/a then bad luck, since you need special hardware to extract the save and hacking tools to hack the save to be compatible with gen 3 or above...
if you got it on 3ds then there is no way you got it without hacking or glitching, you could still wait for the VC release and for what the community finds about the stats of the celebi given by nintendo, then by hacking/glitching make your celebi both shiny and compatible with those stats, and that should give you the ability to "legitimately" transfer it
All youtubers you see either lie or use hacking tools to convert saves