When I search sth or enter web address in safari, a new window pops up.

Are you getting popups? More information would help.

Safari – Popup takes over

Safari – Popup takes over (2)

Popup remover – Scam Zapper

If the pop-up reloads, force quit Safari (command - option/alt - esc) and reopen with the shift key held down. If that doesn't work, disconnect from the Internet, go to Safari/History and delete the website. Quit and reopen with the shift key held down. Then reconnect to the Internet.

Safari -Tech support scam pop-ups

The new window appears with the results of that search or web address, or something unexpected?

In Safari Preferences, what is set for Open pages in tabs instead of windows in the Tabs panel? I have mine set to Automatically so I get tabs, not windows.