Drupal 6.x

With the announcement that support for D6 is ending soon, I have to seriously plan for a migration now. I've avoided upgrading to D7 but will not go straight to D8. I'd like to get some general guidance before I go too far.

(1) I run a custom theme on my D6 site. Will this migrate to D8 or will I need to redesign it for D8?

(2) I make some use of Chaos tool suite, Drupal Wiki, Forum and Advanced Forum modules. Are these all migratible to D8?

i have migrate mysite drupal 6 to drupal 7 and add a file field in one content type, file field is not working it may error occured not create field and also content type, now how do i fix this error?

What exactly happens behind the scene when we generate one time login URL or use Drush uli.

Does it make a database entry with the generated key and then matches this with the key at the generated URL ?

Just curious to know how does it works.

I am not very well versed with Drupal, but one of our site is made in Drupal(by our previous employee): http://www.intigrow.com/iG/DMS. It was not upgraded since 2013. So I took a backup of DMS through FTP and upgraded it. After the upgradations, I tried to open the site but, it is not showing anything. Just a blank page, no Login page, neither showing any error. I tried other paths too, but none of them are opening.

Hello,

I own 8 domains all running one drupal page, but with different content. (languages :-) )
www.website.com
www.website.nl
www.website.co.uk
www.website.etc etc etc

I've just bought the domain
www.website.ro