What happens with phone with no SD storage (only GoogleDrive) when you don't have cellphone/internet signal? And how much data does transfer takes?

I have a S8 android phone with microSD card where all my pictures are stored. Now I need to change phone, and I've just learned the recent phones don't have microSD slot and all goes to Cloud Storage like Google Drive. Except... Isn't Google Drive limited by, among other things, your capacity to access the Internet? What happens if you take a lot of pictures or videos somewhere where you don't have signal/internet reception? Does it not save the pictures or the videos? Furthermore, does the transfer of videos eat all of your data when you get signal back? Does it slow down the phone if you have shitty reception and you get off-on when it's trying to upload? The sellers in mobile shops were just able to answer 'It works.' and I need a bit more specifics... For information, I often need to go to places with no phone signal, that's why I have concerns.