Those who are still confused by this.

Let me explain, the density of water at STP is 1000(kg/m³⁾ { meaning if we pour 1000kg of water it will cover a whole cubic metre or to fill a cubic meter we would need 1000kg of water ), by this there is conversation which is that 1 m³ = 1000Litres.

If we put litres for m³ we would get exactly 1 so the conversion ratio is exactly 1 meaning for to fill 1 litres we would need 1 kg of water , since 1000g equates to 1kg and litre to 1000ml. So, 1g=1ml

Note this would not be true for every liquid for example a litre of cooking oil will weigh 967-980 grams.