fan won't turn off

The fans run all the time by design. They have sincethe first Macbook Pros were introduced in 2006.

Normally, the default fan speed of about 2000 RPM (model-dependent) is so quiet you can only hear it in a very quiet place. Ambient room noise in most setting is far louder than the fans at minimum speed.

The fans are controlled by a thermostat system. If the computer's internal temps approach dangerous levels, the system signals the fans to run faster. At that point the fans may be audible over ambient noise to a person with normal hearing acuity.

Internal temps and, therefore, fan speed can be affected by many external factors as well as tasks. These Apple articles discuss usage patterns and conditions that can affect the temperature and this the fans speed:

About fans and fan noise in your Apple product - Apple Support

Keep your Mac notebook within acceptable operating temperatures - Apple Support

I would add that videoconferencing and any other video-generating activity WILL run up the temps and cause the fans to speed up. So can useless anti-virus and so-called "cleaning" apps. Chrome uses a lot of hardware resource and can run up temps.

It the fans run audibly all the time, especially just after a cold start, the System Management Controller that, among other things, is charged with temperature management, may need to be reset. See:

How to reset the SMC of your Mac - Apple Support

⚠️ IMPORTANT!!! — All 2019 Macbook Pros hav ethe T2 security chip, so be sure to only use tha tpart of the instructions.

IF these do not help, please post back with more details about when this happens, if it is different when using the battery v. the charger, and what tasks the computer is performing at the time the fams are loud.

Try running the Apple Diagnostics to see if any hardware issues are detected.

As @Allan Jones mentions, the majority of times these issues are caused by software (usually third party software such as anti-virus apps, cleaning apps, or third party security software).

Make sure to have a good backup since it is impossible to recover data from a 2018+ Mac if things go wrong with the hardware (and sometimes if it is a software issue).

You should always have frequent and regular backups of your computer and all external media (including the cloud) which contains important & unique data. This has never been more important than when using SSDs and USB-C Macs.