Why has Yoast changed WP site address?

I’ve built my mum a WordPress site using as far as I’m aware the block editor. I wanted to add meta-descriptions etc and remembered using Yoast in previous jobs and it being simple but never had to set it up. Once I installed the plug in, the URL of my website changed – why is this? Is this normal? Can I change it back? Will her visitors see this? I’m stressing because it’s my mum’s site… any help would be greatly appreciated. The new address has wpcomstaging in it ???