It was stated in your video that earth is full of electrons . still when we touch a negatively charged body the electrons transfers from us to the earth but why the electrons already present in earth do not repel the coming electrons .
If the earth is a bit negatively charged, it is understood that it can take away the positive charges of a positively charged body when grounded. Why does it take away the extra electrons of a negatively charged body when grounded?
This is because of difference in charge density (charge per unit area) between the charged body and charged earth. The charge density is always more in the body than in the earth and hence charges always flow from the body to the earth - be it +ve+ve charge or −ve−ve charge.
The low charge density of earth is because of very high area compared to the area of the body being earthed.