Today, I finally visited Duesseldorf and I liked it very much.

This isn't incorrect, but it sounds more natural to include the second "I".

I also saw many Japanese people in the city, andbut I was not surprised because I knew that a small Japanese community lives in Duesseldorf.

I would suggest using "but" instead of "and" as a form of contrast. You make a statement that sounds a little surprising (Japanese people! In Düsseldorf!) and by using "but" you can better express that this wasn't unexpected.

However, it maythis might not be true for everyone because some of my friends would alwayshad a lot of complaints about them.

The second half of this sentence is a little awkward. I tried to think of how I would express this sentiment, and came up with this.

I can say with surety that the first place goes to Duesseldorf amongDuesseldorf is my favorite of these cities, but I know I have to explore more.

Today, I finally visited Duesseldorf and I liked it very much.

I also saw many Japanese people in the city, andbut I was not surprised because I knew that a small Japanese community lives in Duesseldorf.

You don't need the "also" here. It feels out place because the previous sentence doesn't have anything you are adding to with this sentence.

However, it maythis might not be true for everyone because, some of my friends would always complain about them.had many complaints.

I can say with surety that the first place goes to Duesseldorf amongconfidence that Duesseldorf is my favorite of these cities, but I know I have to explore more.

Surety is a very uncommon word, a native speaker wouldn't use it here.

Today, I finally visited Duesseldorf, and I liked it very much.

The parallel structure that you introduced is “I finally v. … v.”, but “I finally liked it very much” doesn’t make sense in this context.

It is a big and modern city with many tourist destinaattractions.

This is because numerous Japanese companies are workinghave offices there.

Companies and organizations don’t really “work,” except in the sense of trying to achieve a certain outcome. Ex: “The University has worked to protect the interests of international students during this pandemic.”

However, ithat may not be true for everyone, because some of my friends would always complain about them.

I also saw many Japanese restaurants in Duesseldorf, and I would like to visit them later.