"Do external usb wi-fi usually include Bluetooth, or do we need to add wireless cards with Bluetooth and wi-fi."  Either way, you can go internal or external.  Example of external WiFi BT adapters.  To answer the first part of your question, USB WiFi adapters are available in WiFi only. Not all keyboards and mice are BT that I'm aware of.  The ones that aren't come with a dongle.
We have a few OptiPlex 5090 SFF being adapted to work from home use. Some of our employees want Bluetooth for wireless keyboards and mice, and wi-fi for home office set ups without access to ethernet connections. What options do we have for adding Bluetooth and wi-fi. Do external usb wi-fi usually include Bluetooth, or do we need to add wireless cards with Bluetooth and wi-fi.
OptiPlex 5090 SFF adding Bluetooth and wi-fi