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

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

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

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

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 saw many Japanese people in the city, but 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.

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 also saw many Japanese people in the city, but 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.

This is because numerous Japanese companies are workinghave offices there. This is because numerous Japanese companies have 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. However, that may not be true for everyone, because some of my friends would always complain about them.

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

However, it maythis might not be true for everyone because some of my friends would alwayshad a lot of complaints about them. However, this might not be true for everyone because some of my friends had 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 also saw many Japanese restaurants in Duesseldorf, and I would like to visit them later. I also saw many Japanese restaurants in Duesseldorf, and I would like to visit them later.

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. I can say with confidence 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.

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