Hi,
If you could export your file so we can look at it we can help you more.
I have watched the video and I agree that the author is good at teaching, however it is also obvious that he is not
trained to design like an engineer, he is designing as a wood worker. This difference in thinking is what is causing
a few issues. He isn't doing anything wrong, just thinking different.
The first thing that jumped out at me in the video is that he grouped his parts but there wasn't an assembly in sight.
It was obvious that he has created the parts in groups as if he was building it. That is not how we usually go about
designing. He actually told you the better way to do the design by using patterns and then ignored it. He would not
have ended up with the mirror problem had he used a pattern instead, OR if he mirrored the SKETCH and then
extruded then used a circular pattern to get the bolts in the right place.
An engineer or designer would be looking for symetry and using it in the design. This cuts down the workload as you
only have to do things once and not repeat yourself as he did with his angle irons. The issue he had was that the bolt
holes were in the wrong place that could have easily been fixed using a pattern tool.

An engineer or designer also would be splitting the design into smaller chunks by using assemblies and
sub- assemblies, which are groups of parts (or components). This also simplifies the drawings considerably as you can
also group things much more easily instead of clicking on many components to select them.
As an example. I would treat the basic frame with the angle irons at each corner as an assembly, and I would treat the
walls on each side and the ends as sub-assemblies. They would then be nicely grouped and I could select each group
individually or as a whole with a few mouse clicks. Doing it this way would also allow me to get my parts list a little
more flexibility as I am using groups not individual parts. I could still create it the way he did, but I could also create it
other ways too.
In terms of Move/Copy/Paste to create the side boards and the floor of the planing box - DON'T. The best way to create
these is definitely with the rectangular pattern tool. You end up with a bunch of individual parts like he does, but
unlike him, if you edit the first one then ALL of the patterned parts will change. If you do it his way, if you move the
bolt in the first one and then copy paste it it will not necessarily inherit the change.
Also, if you are going to use copy/paste then you should be using Joints to put them where they need to be and NOT
placing them by hand. There is no precision in doing it this way. Sure it can be done but it is definitely the hard way.
You always want to work smart not hard.
Drop a copy of your file here and I am sure we can help put you on the right track.
Cheers
Andrew