Upgrading Drupal

I used migrate module to convert my site from Drupal 7 to Drupal 8.9.x. The content types and contents got migrated correctly. However when I try to create a view using a specific content type, I am unable to see the fields. Explaining the steps i followed:

Hello 

I have a news page with filter where user can choose news by category, month and year. By default user can see all newses. 

After upgrading to 8.9.13 i got an error: InvalidArgumentException: The timestamp must be numeric. in Drupal\Component\Datetime\DateTimePlus::createFromTimestamp() (line 201 of core/lib/Drupal/Component/Datetime/DateTimePlus.php).

Hi,

I'm trying upgrade my drupal core from 8.5.5 to 8.9.13 but I having an error with the 'image' plugin.

I'm working with multisite, each site have own your database, for the first database all update were done successful, but to the others I get this error:

I upgraded a site to Drupal 9.1.3, but for some pages, the browser shows this error:

Error: Call to undefined method Drupal\node\Entity\Node::url() in mirum_preprocess_views_view_fields() (line 313 of themes/custom/mirum/mirum.theme).

Hi,
I am trying update my Drupal and get following errors.
Please advise how can fix "require behat/mink-selenium2-driver ^1.4 " and "require ext-curl"?

I have pored over all the docs I can find, and understand that D7 must remain in place so that it can be migrated over to a fresh install of D8. Got it. My D7 is in my public HTML root. Obviously I must install D8 somewhere else. Created a subdomain under the root (so it can be opened in a browser), and unzipped the D8 files into the root of that subdomain (cleverly named "D8").

Nowhere can I find in the docs the best procedure--once the migration to D8 is completed--to remove D7 and go live with D8. Do I just delete all of D7 and COPY all D8 files into the root?