Menu setup. Changed? Not working?

[TopNotchVacs]

I have been away from my site for a while.  Rejigging it. In brief am confused about menu usage.

There seems to be some usage changes over last yr or so I guess.

Any tips are appreciated. Please..dont quote the instructions outta the book.  Its too general. Ive read those.

I don't understand the relationship between menu setup with Navigation and menu changes or selection thru Customize/Header . This is a bit different than I knew

I have a few normal menus setup with Navigation.. Some with subs.. This worked fine. All good.  Lately, been tryin to find a way to hide some menus,,,show others.  Ok so now, no menus are showing. Not sure what I did. Stuff is still ok in Navigation with Menus...menu items, collections etc

I was doin stuff in Customize/Header/ Menus. Button for Select Menu. But I dont understand this.

It looks like you can only "Select" 1 menu here... One??!! For display??

But I have 6 or 7 menus setup in Navigation...some with subs. I dont see a way to show or not show some or all of these.  I dont remember this being such a pain a yr or so ago.

What am I missing? How are these related. !!!