Every Google account offers free 15GB storage for Gmail, Google Drive, Docs, Sheets and other files. Google Photos app is exempted from this storage quota. This means all photos and videos can be backed up automatically for free without worrying about running out of storage space. So, even if your 15GB free quota gets filled by email and Google Drive files, you can still continue to backup photos and videos.
However, this is set to change in a big way. After June 1, 2021, Google will provide a combined free storage space of 15GB for all services including Photos. As photos and videos tend to consume more storage space, you will run out of the free 15GB space quickly and may have to opt for a monthly or yearly subscription plan of Google One.