The shapes and lines shown in your build ups of LEGO models can be results of many kinds of geometric shapes, alone or in combination. From quirky, jagged recessed shapes to volumes built up to be repeated triangular outcroppings, seeing the lines and forms of these in your model builds is a unique characteristic of building with LEGO's various pieces. You can use LEGO's many forms in combination to create whatever kind of positive or negative spaces you can imagine.
Try drawing out various shapes on paper and practice recreating them from the LEGO pieces you have. Or look online and find out about even more shapes of pieces that are available to you. It is an ongoing challenge for LEGO modellers to make tougher geometric shapes to showcase within their builds. -BR