Also, 'they' is used for plural, but not only. I promise you, everyone here uses 'they' singularly all the time. Like many words with multiple meanings or homonyms, we simply learn to understand it within context, as well as writers using context to make it unambiguous.
'They' definition:
1. used to refer to two or more people or things previously mentioned or easily identified.
"the two men could get life sentences if they are convicted"
2. used to refer to a person of unspecified gender.
"ask someone if they could help"