How are they doing it!!!

It takes months. There's not really getting around it unless you farm feathers for hours and hours every day or throw tons and tons of orbs at a banner on which they get featured as a 5* (i.e. costs a lot of real-life money)

There isn't any short cut, you just have score decently in game modes that offer feathers as a reward or grind HM.

the only trick is $$$.
HM Grinding and sending 1000's of Odin's home for feathers.
Pulling stones for the desired units. +10 easy.

Most of my 5*+10’s were all made with feathers, I only have one 5* exclusive at +10 from spending orbs. The key is to do your dailies, quests/missions, pretty much anything to do with feathers. My barracks limit is 500, when I fill it up I go through and send about 100 units home that I don’t want to build and don’t need for skill inheritenace. Other than that, just focus on merging one unit at a time and stick with it.

A shortcut is by using the SS tier item. The credit card.

The answer is $$$

The only way to even come close as F2P is to commit to a certain set of characters. Commit to spend ALL of your feathers on a single build and merge project unit for months. Don't say "hmmm, Lilina has a new weapon. I should promote her". Commit to your core units and promote them to +10. Don't blow a couple hundred orbs trying to get some spring alt with tig ol bitties. Commit to saving your orbs for 6 months until you have 2000 of them and then +10 a character like Hardin or Fjorm who is a top unit and universally useful.

You can max out all your heroes' HM in GHBs. If you use an auto-click program then you can do it easily, but it's technically bannable I think. No one has gotten banned for it as far as I know though.

We’ve been getting a lot more feather with all the new game modes so just save Till you get 220,000

It's because IS has added in many more ways to get feathers than in the beginning months ie: arena feathers

I think a lot of people are exploiting the GHB. It is really only worth trying for those that automate it somehow. But I would imagine those people run the risk of getting banned? Not really sure.

Wow that's my Soleil lol. But to answer you question it's just to save up all copies of a unit you want to 5 star and save up feather by doing all the activities that give feathers. I'm no wail I just play the game alot and the Soliel too k me like 6 months. I've been save copies of her since her debut banner where I pulled 2 of her