Gattz13: Settlements in general work (see screenshot)
if your settler are leaving, make sure to have excess food, water, security, beds.
As for the bridge settlement, steer clear from it, it is kind of bugged.
Happiness depends on above mentioned factors, and despite I tried to boost everything in my settlement, happiness still sometimes declines (but it above 75%, which is 'in green' numbers, tho)
Oh, never forget to set up a radio beacon.
Then a scavenging post + assigned scavenger (+ required perk, to setup trade routes -> which will easily mitigate deficit in food and resources in general, especially handy for crafting, having your settlements connected)
Tip: when your base has a need for 4 food (having food <4 makes the icon RED), install more, ideally twice as much.
So plant food >= 8 (do not stop with 4 or 5)
People usually react a bit late. So if you click on someone, then "assign to" and then clicking on food or whatever in your settlement, give the poor soul like 1-3 minutes to react, before she/he even starts walking towards dedicated destination.
Usually I don't care, I assign them and forget about it, as soon your base is in motion (with working beacon) it will somehow auto-work out eventually.
https://imgur.com/SYWpP8d Cheers. Still struggling to get it to work right. I assign settlers and then they walk out of the settlement. Happiness is at 69. Way more food beds and security (like 5 times the number of settlers.