Please forgive me but I am incredibly new to warhammer and the other answer didn’t really make sense, could I mix Drukhari and Harlequins?
Yes, but thinking in terms of "races" isn't the determining factor (it effectively is, as far as I know, but it isn't the language used by the rules).
In 8th edition 40k, there are three styles of play: Open, Narrative, and Matched. In Open and Narrative, you can just bring what you want and play. Though there are additional rules that can be added which may bring restrictions (like detachments). In Matched, you are required to use the detachments rules, where most detachments require the units in the detachment to be from the same faction. Factions are just keywords that are given with a unit's datasheet.
Since you're asking Drukhari and Harlequins (and I happen to have reference materials for them):
- Harlequin's Troupe unit has the Faction Keywords "Aeldari", "Harlequins", and "<Masque>".
- Drukhari's Kabalite Warriors unit has the Faction Keywords "Aeldari", "Drukhari", and "<Kabal>".
Since both these units share at least one Faction Keyword ("Aeldari"), they can be included in the same detachment.
If you are playing with the armies' codexes, each faction benefits substantially from being in a detachment with ONLY units from that codex (and potentially even more specific than the codex). Each codex has options for sub-factions that give additional abilities to your units, you probably noticed the Faction Keywords that had "<" and ">" above, these are to determine your units' sub-faction (Drukhari are especially complicated, as they have three types of sub-faction, "Kabals", "Wych Cults", and "Haemonculus Covens").
Per the 2018 Big FAQ 2: The Battle Brothers rule was updated to "All of the units in each detachment in your Battle-forged army must have at least one Faction keyword in common. In addition, this keyword cannot be CHAOS, IMPERIUM, AELDARI, YNNARI, or TYRANIDS, unless the Detachment is question is a Fortification Network. This has no effect on your army Faction."
This means that two detachments with the AELDARI keyword can be included in the same army, but the AELDARI keyword is no longer enough to include units into the same detachment.