Men use the masculine form of their family name. Women use the feminine form.
For example, Amara Valda‘s father is Marcellinus Valdus.
And it gets a little more complicated when we get into groups of people and/or grammatically irregular nouns.
But we can fall off that bridge when we get to it.
In general, “-us is masculine; -a is feminine” is usually true.
But -us and -a aren’t the only possible endings for Latin words, and there are words that end with -a that are grammatically masculine — “agricola” (“farmer”), for example, and other words that are professions or occupations.
Women in Rome weren’t usually allowed to work…
Look, I could get into a whole big thing about Latin grammar and Roman naming conventions, but we’d be here all day.