Export the database (all piwigo tables) on A, import on B.
Use either mysql command line or phpMyAdmin or whatever your hosters on A and B provide and you feel comfortable with. Or ask your hosting providers.

Copy all piwigo installation files including upload/ and galleries/ subdirectories (or wherever you keep images when using ftp sync) from A to B. Omit the _data/templates_c/ subdirectory content with generated and cached data that contains path information, or delete it on B. Content will be recreated later when running Piwigo on B.

On B edit local/config/database.inc.php to adjust the database settings as necessary.

That should be it.

If you have only physical albums, install a new Piwigo with fresh database on B, copy only the album hierarchy from A galleries/ to B galleries/, do an ftp sync on B.

If you have physical albums and using ftp sync I answered the question above, under the galleries/ directory.
If you have virtual albums (like images uploaded through the web interface or upload tools using the API) the image files are stored under the upload/ directory. However, in that case all associations from images to virtual albums are only in the database and you'll have to transfer the database as well as recreating it from scratch is not possible.

For Contact there's [extension by Piwigo Team] ContactForm, I never used it. The About text is displayed from the languages' about.html, there's [extension by ddtddt] PersoAbout to add a personal section. The "Now you see it..." is just a paragraph (<p> element) in the page banner, Admin -> Configuration -> Options, tab General, field Page banner.

Check your PHP limits:
max_file_uploads
post_max_size
upload_max_filesize

Probably max_file_uploads is 30.

How do you make pictureA as the default photo in that album
?

VacationMemory wrote:

Where do you find these entries? Under which folder in File manager?

They are PHP settings (which of course are in one or more php.ini files). You can view the settings in Piwigo Admin -> Tools -> Maintenance, under Environment, click on the PHP show info, which in the first section also tells what configuration files are used. To be able to change settings you must be allowed to do so; if the settings contain a non-empty user_ini.filename (usually with .user.ini (note the leading dot)) you can create a file with such name in the piwigo directory and override settings there, otherwise you'll have to ask your hosting provider.

How do you make pictureA as the default photo in that album

Install/activate [extension by Piwigo Team] Admin Tools and view desired picture as Piwigo admin, in the top navbar click Album thumbnail.