Hi, @Sajith Gopalakrishnan Hema ,
To hide these buttons, you can create a application customizer to inject CSS or JS to hide the element. Here is a tutorial about how to do it.
For the return to classic link, you can only disable/enable it on tenant level. Use the PowerShell cmdlet
Set-SPOTenant -DisableBackToClassic $true
If you get error saying the parameter does not exist, you many need to update your PowerShell to latest version.
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