Check out example codes for "flask ico". It will help you in understanding the concepts better.

Code Example 1

<link rel="shortcut icon" href="{{ url_for('static', filename='favicon.ico') }}">

Code Example 2

#the favicon needs to be 16x16 pix
<link rel="shortcut icon" href="{{ url_for('static', filename='favicon.ico') }}">

Learn ReactJs, React Native from akashmittal.com