How to remove Big Sur from a freshly bought mac and install Catalina?

Firstly you need to confirm if the new Macs are M1s, as M1s are only compatible with BigSur. Otherwise you can create a Catalina USB installer by following these instructions https://support.apple.com/en-us/HT201372, then boot from the USB and install Catalina.

sorry, my fault. it's working for T2 as well.
cmd+R -> Startup Security Utility -> Allow Boot Media

Have you successfully installed Cataline on Intel Based Systems that had bigSur preinstalled ? we hear that its not working wiping the disk and using usb Catalina?

Followed these steps but still not successful. USB volume stops booting with the dialog, "Cannot Verify Boot Volume".
Any ideas"

I spent a week trying to figure this out. I believe the firmware locks you out of being able to do this.

If I'm not mistaken I second @Gascolator . We have had hit or miss success with downgrading preinstalled versions of macOS. I believe there is a firmware component that some Macs will have and that percentage will go up as time goes on. Apple probably has a newer firmware tied with the Big Sur installer they are using, and most likely have different versions of the OS installer incase something brakes.

We received 6 Intel Mini's last week with Big Sur, we could get 4 to downgrade to Catalina, 1 refused to let us install Catalina, and the last bricked in the attempt. Apple has never been enterprise friendly here. Though I am shocked they waited until May (6 months after Big Sur launched) to start preinstalling it on Intel devices.

The reason new hardware won't run a macOS version older than the one it ships with is hardware drivers.

While Apple may make the processor chips, it doesn't necessarily make all the other hardware components like the Wi-Fi and Bluetooth radios, graphics cards, USB chipsets, etc. All those hardware components require software drivers and quite often those hardware components are coming from multiple third-party vendors. The same model Macs may have very different internal components.

Hardware components improve or changes over time and drivers need updating. Unlike other operating systems like Linux or Windows, Apple provides those drivers as part of the macOS itself. It doesn't provide them standalone. But Mac users will never have the experience of having to figure out what internal hardware they're running and then see if there are new or compatible drivers for whatever unexplained events they're experiencing.

Apple doesn't retroactively add those newer drivers to older macOS versions, which is why you often can't downgrade. That's the tradeoff Mac users accept for never having to manage drivers.

@talkingmoose - So you are correct about the hardware difference. I was able to create a bootable Catalina installer by creating it from the new hardware. The above links/documentation work perfectly, you just need to run them in the new machines.