How are transmutation circles built?
Almost every symbol, transmutation circle, and array in FmA;B have meanings behind them, even the ones who only appear on screen for a few seconds, like Riza's tattoo (here's a link to a really good analysis of it and here's a video where I talk a bit more about it). All the gates shown also have personal meanings to the characters (I also talk about that a bit in the video). Most of the symbols are based on basic alchemy and I found out a lot of things by simply googeling "alchemy symbols"; so yes, I would say that it is totally possible to make your own circles by combining the diffrent symbols.