This is quite clear and very specific
You are entering Italy from…
Your answer: "UNITED KINGDOM"
Where have you been in the 14 days prior to your attempt to enter Italy?
Your answer: "UNITED STATES OF AMERICA"
Are you a citizen of the European Union (Italian citizens included) or of a Schengen Country or their family member?
Your answer: "no"
Are you a holder of long-term residence status in Italy or are you a family member of the latter, also resident in Italy?
Your answer: "no"
You are allowed to enter/return to Italy from or after a stay/transit, in the previous 14 days, in the United Kingdom of Great Britain and Northern Ireland (including Gibraltar, the Isle of Man, Channel Islands and British bases on the island of Cyprus), but you are subject to specific containment measures, indicated below.
In accordance with Order of July 29, 2021 adopted by the Minister of Health, in force until August 30, 2021, the United Kingdom of Great Britain and Northern Ireland (including Gibraltar, the Isle of Man, Channel Islands and British bases on the island of Cyprus) falls into List D (click on the blue button on top of the page for the complete Lists of Countries and Territories).
Travellers coming from the UK or who have visited/transited the UK in the previous fourteen (14) days, must:
Fill out a digital Passenger Locator Form and present it to the carrier or any other authorised person, either printed or on one's mobile device.
Present to the carrier, or any other authorized person, a document proving the negative result of a molecular or antigenic test, carried out by means of a swab, in the 48 hours before you arrive.
Irrespective of the result of the test mentioned above, self-isolate for a period of five (5) days, informing your local Health Authority in order to activate health surveillance procedures.
take a second test at the end of the 5-day self-isolation.
Children under the age of six (6) are exempt from the requirement of a molecular or antigenic test, but must still self-isolate.
Border Officers in Italy may always request that you fill out a paper-based self-statement upon arrival.