Update 2
I think my confusion came because I did not account for the fact that my project django_project3 is the default project. 
Update
I looked at Using {% url ??? %} in django templates but it didn't fully answer my question.
Original Post
I tried to Google my question but ended up with 1000s of pages of Python theory and no straight answer.
I have a project called Blog and in it there is a file called base.html. And it contains the line of code  <a class="nav-item nav-link" href="{% url 'profile' %}">Home</a>. 
But in Blogs urls.py file there is no URL pattern called profile. 
Whereas in my project django_project3, in the urls.py file there is a URL pattern called profile. 
Does the percentage sign get the code to look in every urls.py file?

 
    