how can i download os x yosemite in bigsure?

Which mac are you wanting to download Mac OS X Yosemite on, model and year.

Some macs that came preinstalled with an OS newer than Mac OS X Yosemite cannot

do the conversion from InstallOSX.pkg to Install OS X Yosemite.app.

If the 'donor' mac is too new to convert the InstallOSX.pkg to Install OS X Yosemite.app

do you have a copy of the Snow Leopard DVD install disk.

If the mac running Big Sur is the right age to create the Yosemite installer app

click on the link below.

How to get old versions of macOS – Apple Support

Go to Download macOS and click on OS X Yosemite 10.10

this downloads InstallMacOSX.dmg to your Downloads folder.

The next section can only be done on a mac that is capable of running Yosemite.

This includes macs that have the potential to run Yosemite but have been upgraded to a newer OS.

A mac that came preinstalled with an OS later than Yosemite will refuse to do the next bit.

When downloaded open to InstallMacOSX.pkg, double-click on

that and an installation window will open, this does not install Yosemite

but converts the InstallMacOSX.pkg to the Install OS X Yosemite.app which 

you will find in your Applications folder.

(If the installation window asks which disk you want to install to, you must pick 

the disk that you are booted to at the time. Not any internal or external disk that 

you want to eventually install Yosemite on, that is for later.)

Create a bootable USB installer disk using the Install OS X Yosemite.app

in the Applications folder and the createinstallmedia command in the Terminal app. 

Read the instructions here,

How to create a bootable installer for macOS – Apple Support

sudo /Applications/Install\ OS\ X\ Yosemite.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ OS\ X\ Yosemite.app