When you store your pictures in your Google account, you have these options to choose from:
"Storage saver" (formerly called "High quality") will:
Resample your images to 16 MPixels, and videos to 1080 pixels when they are larger.
Note that this does affect high quality (4K) videos more than photos.
Apply compression to reduce the storage needed on the Google servers.
See https://goo.gl/photos/waz39uinExJCaGjPA for the effect of compression on size/quality.
Remark: photos of some camera's are not compressed due to the presence of a MPF tag (Multiple Photo file), for example because there is an embedded small JPEG. See this thread.
"Original": this will upload your images at original resolution/quality.
In some countries there is also "Express backup" resizing the photos to 3 MP and videos to 480p.
Very well suited for viewing and sharing, and saving more storage.
You must set the size/quality in each app used for uploading, that is for example "Backup" on each mobile device. The setting in https://photos.google.com/settings is (only) valid for manual uploads.
Till June 1, 2021 uploading with "High quality" or "Express" was free. After that date the actual (reduced) size does count for storage.
After uploading in "Original size" it is possible to use "Recover storage" in https://photos.google.com/quotamanagement
You should recover or buy more storage when your free storage gets full. Google gives you 15 GB free storage and it is shared across Google Drive, Gmail, and Google Photos. You can check following sites for info about used storage:
https://one.google.com/storage how much storage is used by Drive, Gmail or Photos
https://photos.google.com/quotamanagement shows large files, blurry photos, screenshots, and allows to "Recover storage" by compressing photos backed up as "Original" to "Storage saver" quality.
https://one.google.com/storage/management shows deleted (= in trash bin), large and other items like "unsupported videos"using most storage.
https://drive.google.com/drive/quota largest files in Drive
See Google Photos "High quality" vs "Original": What's the difference and should you care
and https://goo.gl/photos/waz39uinExJCaGjPA for examples of compressed photos