i reset my home screen layout, how do i fetch the whole think back

Not at all, if you reset your screens to the default and now you don't like that and want to put it back like you had it there is no revert button and the phone does not memorize the previous layout. So if you want to have it appear as it was prior to going to the default you will have to manually put everything back where you had it.

You can restore your iPhone 12 from a backup of your iPhone 8 and it will use the page layout from the 8, but you will lose any data added since the date/time of that backup. I have no idea what suggestions your provider had you try so I have nothing to add regarding Siri suggestions.

I'll stop here, you said all they did was the screen layout.

I suggest you post a new thread here covering only your Siri suggestions issue and add as much detail as possible including EVERYTHING you have tried so far. Good luck.

Move everything where you want it to go.

Have you gone to Settings > Siri & Search and made sure the suggestions settings you want are turned on?