Where does "Included in Block" value comes from?
I was wondering, how is this attribute populated?

The site uses an index that simply remembers this information. If a block B is seen that contains a transaction T, an entry is stored in the database that signifies &quot;transaction T is in block B&quot;.

by looking at transaction inside of your UTXO, check in which block transaction was included (and then I found this attribute)

The UTXO set does not contain this information, nor can it be efficiently requested from arbitrary nodes in the network.