From some experimenting that I have done so far I can see that copy a web page or WP page and paste into new WP page works but only sometimes. Often it breaks formatting of original page - perhaps due to missing CSS But sometimes it does work. Any guess why or why not?
Since you work on WP why don't you work on themes? Just download the themes you want to learn from and dissecting them into pieces. Do you have virtual host installed on your computer?
If you meant to duplicate an old wp site to a new site. Files and a directory that important in WP: wp-config.php wp-content directory the database file Download wordpress > deploy inside your root directory > delete the original wp-content directory > copy all that important files and wp-content directory into root directory > make a new database > import the old database file.