Open Google Photos on the source account
Select the photos and videos you want to transfer
Click Share
Create or choose a shared album
Add the destination Google account to the shared album
Open the shared album on the destination account
Click Save to library or Add to library
To move everything, use Google Takeout on the source account
Export Google Photos data with Google Takeout
Download the export files
Sign in to the destination account
Upload the downloaded photos and videos to Google Photos
If needed, use Partner Sharing to share all photos with the other account
On the destination account, accept the Partner Sharing invitation
Turn on Save to your library for shared photos if you want copies stored in the destination account