When entering the US, a customs officer fills out your I-94 form.
Anyone who is not a citizen of the USA is obliged to record the date of entry into the country within 72 hours of entering the USA, which is crucial information for the online database of the immigration service in the USA.
The I-94 form is correlated with your J1 visa status.
IMPORTANT NOTE: While you are still at the airport you need to connect to the airport WIFI and check if everything is fine with your I-94 form via the following link: