Upgrading Drupal

Just upgraded a local site to Drupal 9.2.6 from the latest Drupal 8 version.

The site was updated to Drupal 9 with Bartik from 9.2.6. Once the site was a Drupal 9 site, I then re-instituted the sub-theme and copied the css files from the Version 8 site into the sub-theme. I was hoping being the core theme for Drupal that styling would be clean after the transition. Unfortunately this wasn't the case and some of the styling had to be tweeked. The most glaring issue is the sidebar is randomly out of position on certain pages.

This is a local Drupal 9.2.6. upgraded from the latest Drupal 8 major version.

Can anyone offer insight on this, the site is throwing the 'Failed to Fetch Available Update Data'.  I followed #3 found at this link  https://www.drupal.org/docs/system-requirements/php-openssl-requirements

I have to two sites using the latest version of Drupal Core 8.9.19 but each one is for different clients.  They are independent of each other being on separate physical servers.  I don't use composer to update the sites and I've been following the steps to manually update https://www.drupal.org/docs/updating-drupal/updating-drupal-core-manually

I'm a non coder, with a multisite D7 setup; now trying upgrade to Drupal 9.

Not quite as terrifying now as had seemed before, thanks largely to Migrate Module including UI. I've done a fair bit with a relatively minor site [leaving just two bigger ones; mostly articles and photos, plus forums]
- and aiming to have each of the handful of sites run off different Drupal installs; I looked for info and found this recommended for Drupal 8/9.

Hi all,

I'm upgrading to Drupal9, and have run into a problem with my code.

Here's an example; In Drupal 8 this works:

Hi,

I'm using this line of code in Drupal 8, but file_save_data is not supported in Drupal 9. I can't find anywhere ab descriptions of what code to use instead:

$file=file_save_data($filedata, $dir."/".$mediafilename, \Drupal\Core\File\FileSystemInterface::EXISTS_REPLACE);

What should i use instead in Drupal 9?

Thanks

Chris