In this guide I will show you the following:
How to level through Exile's Reach as Horde/Alliance and how to start BFA as both of these factions.
I'm going to leave a text guide for exile's reach that is extremely detailed here if you prefer this over the video guides. (I will put timestamps from where you start the BFA questlines in this guide.)
NOTE: you need to turn on "Trivial Quest" tracking else you won't be able to see the quests after Exile's Reach. You right-click on your minimap and click on "Trivial Quests".

I'll add the quests with their Wowhead links so if you continued to level elsewhere you can find the needed quests.
Horde:
1) https://www.wowhead.com/quest=60343/welcome-to-orgrimmar
2) https://www.wowhead.com/quest=60344/finding-your-way.
3) https://www.wowhead.com/quest=60345/license-to-ride
4) https://www.wowhead.com/quest=60346/whats-your-specialty
5) https://www.wowhead.com/quest=60359/home-is-where-the-hearth-is
6) https://www.wowhead.com/quest=60360/an-urgent-meeting

Alliance:
1) https://www.wowhead.com/quest=59583/welcome-to-stormwind
2) https://www.wowhead.com/quest=58908/finding-your-way
3) https://www.wowhead.com/quest=58909/license-to-ride
4) https://www.wowhead.com/quest=59586/whats-your-specialty
5) https://www.wowhead.com/quest=58911/home-is-where-the-hearth-is
6) https://www.wowhead.com/quest=58912/an-urgent-meeting

Video Guides for Both Horde/Alliance through Exile's Reach all the way to BFA.
Horde
Alliance

Timestamps to skip Exile's Reach:
Horde: Skip to 36:54
Alliance: Skip to 41:41