No difference between USB 2.0 and USB 3.2, why? I have a doubt about USB type declared by smartphone manufacturer. 
Some smartphone declares USB Type C 3.2, for example Galaxy S20 Ultra.

The new Xiaomi Mi 10 Pro is declared as USB 2.0 on Type C. 

I try to check the real connection, and when I connect to my Mac the Samsung I have 480 Mb/s negotiation.
Looks like even if Samsung declare USB Type C 3.2 the max data transfer is 480 Mb/s, and I can prove it with a benchmark.

I connect the Xiaomi, the same. The Huawei P30 Pro, the same. 
Is not the cable: I use a very good cable and when I connect my Oculus Quest to the same port, the connection is negotiated at 5 Gb/s.

So my question: why a lot of smartphone manufacturer declare USB 3.2 port but the data transfer is capped to 480 Mb/s?
tl:dr; You aren't doing an apple to apple, real-world comparison and missing out on many other factors

You are looking at speeds at the USB controller end on PC, further, it is shared between the ports attached to it drawing incorrect conclusions (because you aren't considering other factors)

Firstly, USB 2.0 is half - duplex, which means that while the maximum speed is 480 megabits per second, the actual is roughly half of that. In practice you won't exceed 240 megabits per second speed

In contrast USB 3.0 is full - duplex so, in theory, you can get 10 times higher performance. In practice you aren't likely to get that because of

Type of memory used: Actual data transfer is a function of the USB protocol AND the memory used (protocol may provide high transfer speeds theoretically but the actual speeds depend on the read / write speeds of memory). You mention three devices - Galaxy S20 Ultra, 
Xiaomi Mi 10 Pro both of which have UFS 3.0 memory while Huawei P30 Pro has UFS 2.1. All other things being equal UFS 2.1 gives roughly half performance of UFS 3.0

Memory and protocol combination





On the far right of the graph, the Galaxy S10e and Honor View 20 provide results indicative of what one can expect from a USB 3.0 and UFS 2.1 hardware combination

To the left of those results, the Huawei Mate 30 Pro and OnePlus 7T Pro indicate peak performance from a USB 3.1 and UFS 3.0 memory setup.



Note that the second set of phones have better standards but poorer performance compared to the first!  In other words, latest is not equal to best

As if this wasn't confusing enough, it is said that faulty MTP protocol implementation is the reason why Pixel 4 phones perform poorly in  the above comparison. For more, see Tested: The Google Pixel series has a strange USB-C transfer problem

To sum it up, consider all factors before comparing and do a real world comparison as explained in link above.