Why is the inside of the Earth hot? How does the Earth's internal heat exist? Is it available from its creation as collision heat? Or is there any other source feeding the heat to the core?
 A: In Earth's crust, 80% heat is contributed by Radioactive Elements and 20% are leftover heat from the planet's creation process. Source: Phys.org 2006 article
As for the core (having temperature higher than that of surface of Sun) and mantle, very little is known. There are leftover heat (which hasn't escaped yet because Tactonic plates act as insulator), heat from enormous radioactive energy and heat due to friction/drag (denser rocks are sinking down which are continuously kicked back up by other processes).
A: 
Heat from radioactive decay contributes about half of Earth’s total heat flux 

and the other half comes from residual primordial heat from when the planet was created according to:
 "Partial radiogenic heat model for Earth revealed by geoneutrino measurements" Nature Geoscience (2011).
A more recent source "Exploring the hidden interior of the Earth with directional neutrino measurements" Nature Communications (2017) pegs the number similarly:

Roughly 40% of the Earth’s total heat flow is powered by radioactive decays in the crust and mantle.

