Can European countries bypass the EU and make their own individual trade deal with the U.S.? I am asking, because for instance Italy decided to join the Belt and Road before other countries meaning that it can make certain economic decisions on its own behalf, is it the case for all economic decisions, because it seems that tariffs are handled by the EU.
They cannot. It's an exclusive competence of the EU. So much so that the point played a major role in the Brexit negotiations. The reason there are two phases in the Brexit deal (leave first, then negotiate a trade deal) is that the EU cannot legally negotiate a trade deal with one of its own members.
That being said, China's Belt and Road in itself is not a trade agreement. Rather, it is China extending loans so countries can build infrastructure that ends up streamlining commerce from China to or through the recipient country.
On paper there's some potential benefit for Italy here if it ends up working in their favor. (That's a big if, but that's a different story.) Rather than containers being shipped to ports like Marseilles or Rotterdam and then to the rest of the EU, they'll get shipped to Italy instead. And Italy would then end up collecting more duties, since EU members get to keep a cut of the duties they levy on the EU's behalf.
Being in the customs union means that all EU countries are obligated to take the same tariffs and apply the same import restrictions.
The European Single Market guarantees free movement of goods within the EU. That means that any wares which are in the EU can be moved within the EU without imposing any additional tariffs on them.
This means that if a non-EU country would be able to negotiate a free trade agreement with one EU member, they would in practice have a free trade agreement with the whole EU.
For that reason, any trade agreements with non-EU countries are the prerogative of the EU.
* (except for a few oversea territories where complicated special customs rules apply)
Not all European countries are in the European Union, Customs union or European Economic Area, so yes, there are several European countries which have complete autonomy to make their own individual trade deal with the U.S. They aren't bypassing the EU because they aren't bound by the treaties of the EU.
The most obvious example would be Russia:
From the wikipedia page on Europe:
Since around 1850, Europe is most commonly considered to be separated from Asia by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas and the waterways of the Turkish Straits.
To appreciate some of the complexity involved with the various bodies related to EU trade, you need complex Euler diagrams like this one:
This diagram may be changing on October 31st, though this particular target has already changed several times so far.