The Serrano chill pepper is one of the ‘classics’ that comes from Mexico and is a great ‘hot’ variety for fresh use. The heat ranges a lot but is usually the next step up from a Jalapeño with a nice bitey heat that comes and goes fairly fast. It is usually a fairly fleshy small chill with a soft skin. It is the one you want to use when making salsa. I find they add a nice reliable heat in a slow cooked beef too.
I have found the plants are usually fairly small but reliable producers if they get enough sun and heat. They generally do not like cold but will survive in cooler climates in the open, if kept on the dry side, unless they get frost which will kill them (totally). Best to winter these bushes under cover or inside a covered green house. Otherwise they are pretty hardy growers.
Serrano, along with Jalapeño, have been interbred a lot to create many variations so it’s always hard to say what heat and size they will be till they fruit. It’s likely best to only select the smaller fleshier thin skinned and hotter ones for seed saving as that is often what is lost in the crossing. Also best to keep them separated from the jalapeños as they will easily cross.