Hello Ornery1,
We'd be happy to provide some guidance. There is a big chance that you're seeing low photos and videos resolution on your iPhone because you have "Optimize iPhone Storage" feature turned on your iPhone. With this feature turned on, smaller, space-saving photos and videos are kept on your iPhone while original, full-resolution versions are stored in iCloud.
To get started, let's check if this feature is turned on by going to: Settings > [your name] > iCloud > Photos.
If you would like to turn it off so you'll have full resolution photos and videos on your iPhone, you'll have to have enough storage on your iPhone. With that being said, can you go to: Settings > General > [Device] Storage, and tell us how much free storage is available there?
If you have enough storage, then you can go to: Settings > [your name] > iCloud > Photos, and select "Download and Keep Original".
Keep in mind that if you don't have enough storage on your iPhone, there is no way to increase it as each device comes with a fixed amount of storage, on the other hand, you can upgrade your iCloud storage if you needed more space on iCloud.
The following links explained how this works, and the difference between the device storage, and iCloud storage:
Manage your photo and video storage
How to check the storage on your iPhone, iPad, and iPod touch
What’s the difference between device storage and iCloud storage?
We hope this information is useful. Kind regards.