Installing MacOS X on partition

I have made a few partitions on my late 2015 iMac 16,1 today. I want to install MacOS X 10.6 on one. I have an ISO file with the app on it, but it has one of the restrictid logo thing. I can't open the app, because it isn't supported on MacOS 10.15. So i want to make a bootable MacOS X 10.6 USB drive (With max 8GB, limited options, i know.) via Disk Utility and willingly using Terminal too. As i said, there was some restriction logo over the icon. I have a duplicate file of the .icns file, but normally as an app it has one of those logos. When i try to start the app, it says: You can’t use this version of the application “Install Mac OS X.app” with this version of macOS. You have “Install Mac OS X.app” 23.1.

Resources i have:

USB sticks: one with 4GB and the other with 8GB.

An ISO file with an installer app unsupported by MacOS 10.15.

An 125GB partition just for it.

A Windows 10 laptop if needed.

Also, i have a dmg file with the same stuff support problem, but there isn't even an original logo. So i trust the ISO one more.

---"Emergency" resources.---

Another iMac 16,1 with El Capitan on it (My sister's, which i'm sure won't let me use it)

Another iMac from mid 2011 with Mojave (My parents', which also probably won't let me.)

---My idea---

Somehow make a bootable USB drive with MacOS X 10.6 on it, for example with the same files as an install CD, boot off of it, and install it on the partition.

Or i can attempt to install MacOS X 10.9 and try to progress from there. I'm more focussed on the first one though.