what is the topic-view page’s name? in bbpress

This will depend on your theme. But for the templates that come with the BBPress plug-in, the single topic template page is singe-topic.php. This in-turn doesn’t do much, but call content-single-topic.php. This displays stuff like the password form for protected topics, displays topic description etc. and then loops through the replies to the topic with loop-replies.php.

content-single-topic.php includes the form to reply to the current topic. If you are after where the form to write a new topic is, that is in, content-single-forum.php, called from within single-forum.php.

These files will be within the plug-in’s folder, unless your theme is ‘bbpress-ready’. You can copy these files into your current theme and then add add_theme_support( 'bbpress' ); into your functions.php to tell WordPress to look there for the templates.- Then you can edit the templates there.