Often a single component (like a potentiometer/trimpot), or multiple components are combined in a certain way, can be useful in many circuits. They will not likely be explained in the explanation of more complicated circuits as it will probably be assumed that you already understand them. If part of a circuit is explained as having an input and/or an output, it is likely a circuit fragment.
I just created this page, but I will use it to make a list of circuit fragments and add a link for most of them to a page that covers them.
– fixed resistor voltage divider
– Potentiometer or trimpot voltage divider
– LED polarity indicator
Remember that many simple circuits are used as a fragment of a larger circuit. I will limit the number of them listed on this page as I am going to make a page that lists circuits I will be covering.