The “firstborn of the father” receives a “double portion” of the “inheritance” as well as “being the leader” of his “brothers”.
However if a “son other than the firstborn” “receives this inheritance”, “he is called” the “firstborn”.
Interestingly this is a “very common occurrence’ within the “Biblical text” such as we see with “Jacob” and “Ephraim”.
Strong’s: #1060 – https://biblehub.com/hebrew/1060.htm