Refer: https://www.howtogeek.com/265027/how-to-disable-cortana-in-windows-10/
You will need to use regedit and you might need to add a new key. Navigate to:
HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\Windows Search
If you don’t see a “Windows Search” key (folder) below the Windows folder, right-click the Windows folder and select New > Key. Name it “Windows Search”.
Right-click the “Windows Search” key (folder) in the left pane and select New > DWORD (32-bit) Value.
Name the value “AllowCortana”. Double-click it and set the value to “0”.
You can now close the registry editor. You’ll have to sign out and sign back in or restart your computer before the change takes effect.