What does a virus actually do to a phone or computer?

virus can be malicious programmed to re-write or wipe out microcode/BIOS information which hardware will malfunction without it

using simple expression consume your resources and it may cause to damage them

A virus can do multiple things to a phone. It can steal information from the phone. It can cause errors on your phone. It can also cause the data to go slower or stop functioning all together. A virus can be very damaging to a phone and sometimes a reboot of the phone is necessary to remove it. Sometimes, the phone will have to be replaced depending on the damage.