From transparent background to white

Hi guys,

I know that this is such a silly question but I just started to use Photoshop and I'm following different courses and I'm having trouble reproducing the exact same result when the background should be white but for me, it goes automatically with the transparent look.

I attach pictures of the course with the white background and mine, is there a way to have it automatically loaded with the flat background when opening a file in Photoshop?

Thanks!