Most of the Bruiser build credit goes to 3 Minute League of Legends on YouTube, as I watched his video to get a good starting point and idea of what items to play around with. So its mostly me testing out different combinations and explaining them in a more detailed guide.
https://www.youtube.com/watch?v=wknI62PJOCo

As for the rest of the assassin builds, I really looked at a few other guides to see what other bruisers got assassin builds and what items helped them become said assasins. The first guide was an Ambessa guide by "CurSteel", which I had used for my Ambessa games before, so I recommend checking his guide out for Ambessa builds.
Another Guide I read was "Ir.Defenders" Xin Zhao guides as the two champs are quite literally similar down to design, but have different ways of executing how they want to go about getting onto targets and dealing damage.

"CurSteel" - https://www.mobafire.com/league-of-legends/build/quot-a-wolf-has-no-mercy-quot-comprehensive-guide-for-all-levels-to-ambessa-646973
"Ir.Defenders" - https://www.mobafire.com/league-of-legends/build/xin-zhaofend-to-the-arena-553218

A lot of bruisers and assassins can be built the same, so it's natural that the builds look very similar. especially since I haven't got to test them all extensively yet, besides the bruiser build. As for the assassin build, I went for more traditional assassin items since I wasn't sure if the outlying items in the Xin Zhao assassin builds would work well for him, too.