Winforms / Understanding Tab Order in VS 2019

I have many winforms forms that were built in VS2019 without regard to tab order. Anywhere from 10 to 300 controls, some have multiple tab pages.
All need to be fixed and I watched several videos and still have no clear picture of how it's supposed to work

My question so far are:

1. It appears that each form does not have an tab order that starts with 0?
2. My main menu form (designed first) does, but susequent forms start at 38, or 500.. seems like starting tab order number depended on the order I created them in?
3. TabIndex property also does not start at 0 per form?
4. I'm sure I'll make mistakes as I go, so how do I fix the one field in the middle of the page that's out of order?
5. How do you reset the tab order altogether?

This is going to be a lot of work, so I need to understand how it works and learn all the tips and tricks before I tackle it.