It took me awhile to get the first nornir example to work since I'm new to .yaml files. Here's another look at the hosts.yaml and groups.yaml files to help those that are in the same boat as me.

Treat the dots (.) as spaces:

-- hosts.yaml --
---
al-rtr-1:
....site: alderaan
....groups:
........- routers
....loopback0: 10.230.12.1

en-rtr-1:
....site: endor
....groups:
........- routers
....nornir_username: wicket
....nornir_password: G0lden_god
....loopback0: 10.230.12.2
....nornir_host: 127.0.0.1

ta-rtr-1:
....site: tatooine
....groups:
........- routers
....loopback0: 10.230.12.3

ta-sw-1:
....site: tatooine
....groups:
........- switches

ta-sw-2:
....site: tatooine
....groups:
........- switches

-- groups.yaml --
---
defaults:
....nornir_username: luke
....nornir_password: blue_saber
....domain: empire.space
....contact: Unknown

routers:
....nornir_username: han
....nornir_password: I_shot_first
....nornir_nos: iosxr
....contact: Router team

switches:
....nornir_nos: ios
....contact: Switch group