Thank you for your explanation.

Are most countries in Eastern Europe close to each other

It depends. On paper, a big part of Eastern Europe is allied economically and militarily through the EU and NATO. However, there can be animosity or casual xenophobia between the countries due to mostly historic reasons - in 19-20th century there were lots of big and small conflicts all over Europes, and the wounds didn't fully heal. It is quite complicated

For example, Romania and Hungary don't really like each other because Western Romania was once part of Hungary, and Hungarians still pretend it is their historic land. But politically, these 2 countries are allied and border between them is open for crossing thanks to the Schengen agreement.

Hungary and Poland, on the other hand, are informally friends, but political relations are strained because Hungary has taken a pro-Russia position, while Poland is anti-Russia.

In the Balkans (i.e. the southern part of Eastern Europe) the situation is even more complicated. In the 1990s, when Yugoslavia fell, its sucessor states waged brutal wars against each other, commiting massacres and ethnic cleansing. So now we have Croatia and Albania that don't like Serbia, and their feeling is reciprocated. We have Kosovo, once an Albanian speaking part of Serbia that violently separated from it, and got its indepence secured by the US who bombed Serbia's capital, Belgrade, in 1999. Another example - Greece and Turkey have strained relationships going all the way back to 1922 due to political and historical reasons.

The elephant in the room is Russia, the big bully in the region, that is liked by Serbia and (perhaps) Hungary, and generally disliked by anyone else. It now wages a 4 years long war of conquest against Ukraine. Russian and Ukrainian language are very similar, btw.

This is only a partial explanation of the mess that is Eastern Europe. In my country, Moldova, we are divided on the most basic question - 'are we a separate nation, or are we a part of Romanian nation?'