How can I change my MacBook’s system?

Just guessing, as I have no experience of this. As an Intel Mac will normally be running Windows in Bootcamp, part of MacOS, the first thing to check is whether MacOS is there in some form. Windows might simply be booting by default. Start up holding down option/alt and see if any other option appears. Start up holding Command+R and see if Recovery is there, allowing you to reinstall the MacOS. If no joy with either, this thread gives an indication of how to produce a Mac USB bootable installer on a Windows machine:

make a macOS usb stick with windows

You will need to download a version of MacOS that is compatible (Big Sur?). Older Mac installers can be found here, as well as instructions on doing the installation:

Installers for older macOS

Is it downloaded from Bootcamp or like a normal windows PC (like you use an USB or methods like this)?

If from Bootcamp:

Windows is an part of your disc where Mac OS is still downloaded. When you start your Mac it will automaticly boot to Windows. If you hold Option ⌥ on start you can see Startup manager / selector of discs. You may see two of the discs. Select one of them. One of them may have Mac OS.