Ok lets go with the basics here.
If your 1000% sure it is white smoke…
Does the cloud, once it exits the tail pipe dissipate fairly quickly or does it linger.
If it dissipates quickly your dealing with burning water or coolant, no question about it.
Where your leaking it is the question.
See if these are of any help.
https://www.mazda6club.com/threads/incredible-amounts-of-white-smoke-from-exhaust.192766/
https://www.thedrive.com/maintenance-repair/37516/white-smoke-from-exhaust-on-startup