Sometimes it makes sense to make part of a model separately a join mesh data together later on, or sometimes it is necessary to join mesh data together for export. Whatever the reason is, you now know how!
I hope you have enjoyed the lecture. If you have any hints or tips that you have discovered about this topic please do share. Feel free to report any issues with the content if you have any!
In the video, you joined a block to some solid rails on either side. Doesn’t that create duplicate facings?
It can do, whether or not this would be an issue depends of the models application. 
Micheal, the challenge at 5:20 requires you to apply the array modifier before joining mesh objects. In blender 2.9, I found out the “apply modifier” button was moved behind the dropdown arrow after a couple of googles. You may want to point this out to other students.