I'm a novice, Jim.
I created a very simple flow in an Excel file last week. It worked.
Today when I open this same Excel file and click on the Flow button on the Data tab, it says "You don't have any flows for this file". This is where the flow was listed last week, but it's gone.

But I found the 'Manage your flows' page for Power Automate, and the flow is still listed (it's called 'Add to listservs' in the screenshot below.).

(note: the Environment is obscured so I don't embarrass my employer. But there is only one environment, and I am in it.)
One question is:
How do I add this flow to my Excel file?
Another question is,
Why did this flow disappear from my Excel file?
(and how can I avoid that in the future).
For background: (1) I tried opening the file in both the Desktop and Online versions of Excel, and the flow isn't in either one. (2) I'm using an "Enterprise" license of some kind, for all O365 products. (3) I'm logged in to the same account in every context; I only have one account.
In lieu of a solution, I'd like a pointer to an explanation of where Flows live and how they are associated with individual Office files. What is 'instant' flows? Etc.