I noticed when Trading with settlers what weapons they have. And that I have the opportunity to give them better weapons and armor. Is it assumed that I am supposed to give each and every one of them better weapons and armor? So that they can fight and survive better when their settlements are attacked. Am I also required to provide each of them with ammo for the transferred weapons? Unlike so many other subjects that come up, I am NOT getting any helpful hint popups on the subject.

[All things considered, my settlers will look kind of out of place when all I have to give most of them is Raider gear and clothing.]

I usually give them some small amount. In previous games, having some ammo in their inventory allowed NPCs to fire unlimited. As to whether you should or not, all I can tell you is that one of my settlements killed a deathclaw within about 3 seconds. They all had combat rifles. So I think it makes a difference.