From where can I download the solar monitoring app to use on my phone?

Most solar monitoring is inverter (or device) specific.  Therefore, you would download the app from that respective manufacturer’s website.

Here are the most common ones.  For a detailed listed, please see the System Info section of the knowledge-base:

Enphase
Fronius 
SolarEdge
SMA
Sunpower

See also:

What is solar monitoring?
My solar monitoring is not working, what do I do?