How to make the option being pre-selected? Or a way to make a select all button?

You can use auto-fill by URL parameter. Check this help docs: https://help.youform.com/p/_9cYLMIzF_7e6p/Auto-filling-via-U… Also, this form: https://app.youform.com/forms/xrjcjyti?prefer=878e3cbe-⁠012b… (Youform will be pre-selected on the 3rd question)