Why would the germans build camps just to kill people?

Because they wanted to kill people duh.

In western Europe, they still wanted plausible deniability and secrecy, and it wouldn't be very deniable or secret if they just went from house to house shooting people on the spot in front of witnesses.

They could get away with mobile killing squads in the Soviet Union, where they didn't care if people found out what was going on. But that was hard on the squads, most people are not psychologically able to murder dozens of unarmed women and children in cold blood day after day, week after week. Even the battle-hardened SS officers complained about having to kill women and children, and sometimes refused orders to do so. Himmler was very concerned about the psychological effect it was having on his men.

And it is very inefficient. The camps were far more efficient. The extermination camps were capable of killing thousands of people a day, as fast as they arrived, using only a relatively small number of guards, and disposing of the corpses. Mobile killing squads could not come even close to that efficiency.

Why did the russians modify the camps?

Which camps? Modify in what way?

Why did the camps have pools?

Do you think that perhaps the swimming pools were for the use of the guards and officers running the camps?

The Germans built literally thousands of concentration camps. At least one camp was a show camp, deliberately set up to fool the Red Cross and dispel rumours of ill-treatment. Some were detention camps for favoured prisoners and detainees who would be deported to countries Germany didn't want to offend, or swapped for Germany prisoners. Many were work camps, and some camp commanders decided on a "carrot and stick" approach to get more work from the prisoners before they died. Some were run by the Gestapo, for interrogating prisoners.

Camps in eastern Europe were generally harsher than those in western Europe and Germany. Camps with only Jews and Slavs were usually harsher than those with prisoners of mixed origin. Some camp commanders allowed their prisoners little luxuries to encourage them to comply with the rules.

Only a handful were outright extermination camps where majority of prisoners were killed immediately on arrival.

Which camps do you think had swimming pools? Can you name them?

Why did the germans allow children to be born in the camps?

The Nazis were in power for 12 years, from 1933 to 1945. In that time millions of women went through the camps. Especially early on, the Nazis were still aiming to deport Jews and they had no reason to prevent pregnant prisoners from giving birth. The last thing the Nazis wanted was the Red Cross reporting that they were not giving medical attention to pregnant women, or for deported Jews to report that the Nazis were allowing pregnant women and babies to die.

Things were very different in the extermination camps in the east, where such things could be hidden.

There are records of about 2000 births in the camps, that's fewer than 200 a year on average, a drop in the ocean.