In order to express that something is only one thing from within a larger category, the construction ～は～の一（ひと）つだ
will be used. In this expression, (A), the thing that there is one of, will be followed by は
. After this, (B) + の一（ひと）つだ
will highlight the broader category to which (A) belongs.
Android is one of (many) smartphone operating systems.
Trains are one of (many) vehicles.
In this grammar point, の一（ひと）つ
is a standard counter. This means that other counters can and will be used, when they are more appropriate than 一（ひと）つ
He is also one member of my family.
Tomatoes are one of many types of fruits.
This crayon is part of this set.