When it's all said and done, it looks like a few (massive) simple pieces of wood connected in the most basic of ways.
What most people don't see, or maybe won't ever see, is the 20 or so hours of work - a lot of which is done with chisels and planes - it takes to get those pieces of wood in place, fit together reasonably well and above all else, secured safely to the wall behind them.
I generally start these projects by first attaching 2x6s to the wall where the surround (the legs) will ultimately land. I then hollow out the legs to envelope those 2x6s and get them temporarily attached. With the legs roughed in, I can then temporarily set the mantel beam in place to gauge - based on what my level and tape measure tells me - how much each leg needs to get cut to 1, allow the mantel to sit at the desired height and 2, allow the mantel to sit level. I prefer doing things this way, instead of doing a lot of crazy measuring ahead of time, because these reclaimed wood beams are never completely square, flat or even a uniform thickness and relatively speaking, all the math in the world can't account for the imperfections and irregularities. Given how challenging the cuts can sometimes be (i.e. and 8" thick beam and a circular saw that only has a depth of cut around 2.5"), it's easier to just get everything close, set everything in place, see how it all fits and fine-tune it from there.
Similarly, I like to use a French cleat system in the horizontal piece - the mantel - and with that pretty much needing to be dead nuts accurate for the system to work, I generally hold off on installing the portion of the system that goes on the wall until I know exactly where it needs to go for everything else to work out right. That French cleat system involves attaching an angled piece of wood to the wall, hollowing out the back side of the mantel and insetting a corresponding angled piece so that one piece slides over the other and the mantel essentially hangs from the wall.
Once everything fits the way I want it to on a dry run, I'll go ahead and throw down a little polyurethane and then use glue and a handful of discrete trim screws to tighten everything up.
Long story short...it's a lot of work, and dealing with super heavy chunks of reclaimed wood doesn't make any of it real easy. But it's fun, and if I've done my job, nobody ever sees any evidence of all the work that goes into getting these things built.