Wifi uses a listen before transmit system. If the channel is busy, it holds off transmitting. Eventually it gets through. Each channel is fixed. If it tries to use a channel that is busy, from bluetooth, then it will wait. This may reduce the data speed for wifi if it has to wait too much.
Bluetooth for over a decade now uses adaptive frequency hopping (Bluetooth 1.2) so it will actively check each of it's channels to see if they are good or bad, and blacklists them for a while. This is on top of it's normal frequency hopping. If it tries to use a channel(s) that is busy, from say wifi, then it will move on to the next and won't use those wifi channels, essentially not interfering.
So both try to actively prevent interfering with others and suffer data loss or speed loss for it. But because of these techniques, they can coexist. In a quiet environment, the wifi and the bluetooth networks won't even bother each other. In a noisy environment, there goes your bandwidth.
Here is a nice if old (2006!) writeup on interference techniques for wifi and bluetooth (and ZigBee and wireless usb) https://www.eetimes.com/document.asp?doc_id=1273359#
And even newer (but still old) the development of all in one wireless networking ICs that handle both Wifi and bluetooth for the same device, allow interoperability by communicating to each other so that they know when the other is transmitting and what channel, so they can avoid each other. Teamwork. https://www.marvell.com/wireless/assets/Marvell-WiFi-Bluetooth-Coexistence.pdf