No, it's supposed to be used just like others, it just has a prefix at its 'base' unit. Milligram and Megagram would indeed be technically right, but no one uses it (edit: in the case of megagram, thanks daveguy).
I suspect this is because, sort of like length, there's many different names for essentailly different orders of magnitude and you'd first go to the largest (a tonne, 1000 kg) before adding on prefixes. I suspect this many-names factor is in turn because they're such historical things that humans have measured and quantified since prehistory, because distance and mass are the first 'quantities' we begin to encounter.
So instead of 793 gigagrams, I'd say 793 thousand tonnes, or 793 kilotonnes.
Milligram is used extensively in biochemistry when defining concentrations milligrams per milliliters or mg/ml. This is the equivalent of g/L, but in biochemistry lab quantities are often measured in mg and ml rather than larger masses and volumes.
Sorry you're right, I added that 'no one uses it' comment on afterwards in reference to the Mega- prefix and in my exhausted state didn't parse the milli- version which indeed people do use.
Per my latter comment, it's because we don't really deal with everyday words for masses smaller than a gram, but we do for those bigger. It still holds that as far as I can think, people will go to the closest 'common term' word and then use prefixes to bridge the remaining gap (milligram, megatonne)
To be clear, a "liter" and a "litre" are the same unit of volume, just spelled differently. (It's not like "gallons", where the US and Imperial gallon have different volumes.)
I do not have any sources, but I'm pretty sure litre (or liter) is used very extensively, across the ocidental world at least, including Australia. What country are you from?
I suspect this is because, sort of like length, there's many different names for essentailly different orders of magnitude and you'd first go to the largest (a tonne, 1000 kg) before adding on prefixes. I suspect this many-names factor is in turn because they're such historical things that humans have measured and quantified since prehistory, because distance and mass are the first 'quantities' we begin to encounter.
So instead of 793 gigagrams, I'd say 793 thousand tonnes, or 793 kilotonnes.