[ubuntu_mate] Can you help me to install Ubuntu for dual boot?

Hi,
so a little background, i am new to Ubuntu, i tried it back then with WUBI and it was easy to install, and ubuntu was an ok system, but at the end i still ended up using Win7.
It's a good system. But now Microsoft is very pushy about Win10, installs telemetry with forced updates on Win 7 and 8 and you can't run DX12 without Win10 and with Win10 you basically are a "glass citizen", since they track everything and all that ********... so i decided, not with me. Through Smartphone Linux becomes more common, Vilkan runs on basically everything, AMDs support is getting better and better, also on Linux based OS' i heard, so, after reading and watching many videos, it looks like Ubuntu MATE and Gnome are the two best distros. But i still, don't want to lose the option of Win7, eventhough i plan ruing also a VM with Win7 in Ubuntu MATE with GPU passtrough for compatibility.

Now the actual questions:
I made a USB stick with MATE, booted from it, as i saw on a explanation video, from the UEFI mode and created a SWAP partition, then clicked to install on the rest of free space, but then the installer asked, if i want to instal in UEFI, because there is a OS detected installed not in UEFI, and if i proceed installing, the other OS will not be able to boot anymore, so i said no, tued off the PC, and to install just Ubuntu i discoected every HDD, including the one with Win7, but the one with free space i left coected, the same message came up, but i completely discoected Win7, why did it show up then?

I can make a video if you need.

Please respond, i've had enought of Windows' shenanigans with Win10...