What is SharePoint Infrastructure
The question is valid and probably not many people would or could answer it appropriately.
In broad terms, SharePoint infrastructure is the composition of all those components that lie underneath your SharePoint web applications. That is; databases, user directories, web server sites and application pools, file shares, routing, switching, firewalls, ports, authentication mechanisms etc. It is also patching, monitoring and backup.
SharePoint Infrastructure (Location on Google Drive)