UWB Indoor Positioning
Are there any resources where I can study about the algorithms it uses to calculate the direction?

&quot;positioning&quot; and direction finding are typically two different things; you can use direction finding with enough observers for positioning, but it's not the only way, and probably not used here. UWB and angular resolution is hard, because the usual &quot;beautiful&quot; algorithms for direction of arrival assume something narrowband.
There's nothing inherently fundamentally different about UWB pulse radar and pulse radar in general. And there's about 116 years of pulse radar history; I'm going to go ahead and assume you're a student; maybe ask your supervisor or professor in a radar or high-frequency lecture what they can recommend; it's probable they have a good idea of what would embed well into your educational background.

Also why does it able to provide a centimetre accuracy?

Radar systems are limited by noise and by bandwidth. Remember what &quot;UWB&quot; stands for!

For my understanding, it sends pulses in different frequencies within the frequency band.

A pulse is a wideband thing, it doesn't have a single frequency; so there's something a bit questionable about that wording.
Now, what could very well be is that it's a stepped-frequency wideband radar.
However, we don't really know which kind of UWB radar you're referring to, so I'll refrain from speculating here – might only confuse the both of us!

I am not quite sure how the communication is established.

Localization or radar are not communication methods.

Is there a specific protocol used similar to Bluetooth?

Almost certainly not.