Level 100 and Magni Bronzebeard

Even though I have a couple of characters to level 100 I still class myself a newbie.
I accidentally boosted my Mage to level 110. So I decided to level another mage and hunter, doing it the slow way, to 100 as I did not wanted to miss all the EXP.

The Mage is level 101 and the hunter 100.

Now I have both of them in Draenor building a garrison and getting
frustrated with all the mistakes in the gameplay, but that is another story.

The actual question is, when does Magni Bronzebeard/Silithus and the heart of Azeroth come in ?

My boosted mage has the Azeroth necklace, or whatever you call it.

Actually talking about the boosted mage she missed out on building a garrison and recruting followers etc. Can that still be done ?

Thanks in advance

Assuming you have purchases the Battle for Azeroth expansion, your 110 character should get an auto-accept quest called “A Dying World” that directs him to go to Silithus. If you hit the world map (“M” keybind by default) and don’t see it at the top above all other quests, go to the Stormwind or Ogrimaar capital city and it should pop for you there.
https://www.wowhead.com/quest=53028/a-dying-world
https://www.wowhead.com/quest=52946/a-dying-world

When you get to Silithus you should see a big massive freaking sword sticking in the ground, like bigger than the Empire State Building massive. Do what Magni tells you to do and you get the Heart of Azeroth necklace. Then you should get another quest sending you back to Stormwind or Ogrimaar and you can begin the Tides of War quest.
https://www.wowhead.com/quest=50668/tides-of-war
https://www.wowhead.com/quest=46727/tides-of-war

As far as getting a garrison, your 110 can absolutely do this at any time. Go to the Blasted Lands zone, and there should be a quest giver NPC right near the Dark Portal. Do those quests, go through the portal, do the WoD starting scenario, yada yada yada, land in Shadowmoon if alliance or Frostfire Ridge if horde and pick up a quest that eventually builds your Garrison. You know have a fort built of sticks.