“But they/them is plural!” This is an argument (or question) that feels real and pervasive. Unfortunately, it is not a grounded in a thorough understanding of English grammar at all! If you need more formal sources (dictionary, style guide, etc), there are more references here–or you can watch some or all of these videos (total 23 minutes of content).
Often, we find that the question (under the question) isn’t really about grammar at all. If what you really want to hear more about is non-binary or they/them gender experiences, then I recommend this other “What Does They/Them Singular Really Mean?” blog entry.
The Comma Queen puts they/them singular in context (2 minutes) after they/their-singular is named “word of the year.” Also, briefly introduces “neopronouns” (such as zie, zem).
This (2 minute) video from Merriam-Webster looks a little more at the “they” plural vs. singular question by exploring alternative like “he”-universal and “he or she” as an indefinite pronoun. More from Merriam Webster: Singular They; Word of the Year 2019 and WHY. This is less about gender and more about grammar–and even references the King James Version of the Bible.
In the next 4 minute opportunity, you’ll get a more linguistic view, which notes how gender functions in language generally. This one sometimes drifts into humor, but it’s accessible and raises important points beyond the singular/plural question.
This video from the Khan Academy (9 minutes) provides a bit more grammatical background on how they/them (and you/you) have evolved in the English language. This is a bit more of an evolutionary, historical look at how the English language has changed over several centuries,
Finally, Riley Dennis spends 6 minutes digging through the grammar, the feelings, and the cultural context.