Sign in to the source Google account
Open Google Photos
Select the photos or albums you want to transfer
Click Share
Create a shared album or share with the destination account
Sign in to the destination Google account
Open the shared album or shared photos
Click Add to library or Save to library
Wait for the photos to appear in the destination account
If needed, use Google Takeout to export all photos from the source account
Download the exported files
Upload the downloaded photos to Google Photos in the destination account