Purpose of hot water tank and boiler
Without seeing your particular system it is impossible to tell how it is configured.

Generally, the device called a boiler is for heating water to heat the house. This is a closed system and does not supply hot water for the house.

A device called a water heater is used to heat potable water for the house that is safe to drink and all other personal needs. This is an open system and takes in cold water from the domestic water supply to heat for use.

It is possible your system does both but the two piping systems should be kept separate so the potable water supply is not contaminated. Your system may pre-heat water while the boiler is running to heat the house and store it in a tank.

To get a better answer you need to post pictures or diagrams of your system.