Some people are completely fine with all possible uses and combinations and you can just use whatever you feel like. Some are kinda fine with both, but actually do have a preference and view the second pronoun as a compromise that’s ok-ish when they’re around random cis people they don’t want to give a ted talk about the complexities of their gender to. Some will want you to use both sets of pronouns alternatingly (“she asked me to come over, but i was busy, so i asked them if we could postpone”). Some (especially genderfluid) people will have a preference that varies over time and may have markers like a pronoun pin or clothing choice to signal which pronouns they go with at a given time, like she / her when in drag and he / him when in masc-coded casual clothes.

The tl;dr is that you really can’t generalize what something like she / they means in practice and that you should just ask the person.

Edit: Also when people are out just recently, it’s possible they’re trying out which pronouns work best for them. In that case, it may be a good thing to help them get a feel for both by using both pronouns in different situations and, if you’re close enough, asking them after a while if they’re still using both sets of pronouns. A lot of people make the experience of introducing themselves with, say “they / he” and everybody just he / hims them 100% of the time, and in that case particularly, i’d ask how that person feels about this when some time has passed and there’s a good opportunity. Such choices do not always work out as intended, and people can and do change their minds about the fine print when exploring their gender.