Drupal 8.x

My site is installed using a 'oneclick' install via my hosting company.

The current version is 8.9.16, php 7.4.29, db 5.7.37-40-log.

It is prompting me to upgrade to version 9.x, and as far as I can see it is either via Drush or Composer.

I don't know if it is installed using TAR, DRUSH, Composer or something else...actually I'm a bit in the dark here.
So my question is, what do I need to check to find out how to update my Drupal?

Hello,

I need to make a field or configuration parameter that is for all content of Article and can be change. I am new at Drupal.

For example, all Article that are posted in last 10 days should display "New". After 10 days pass Article doesn't show "New" on display.

And the most interesting part is that 10 days is not fixed, I need to be changeable. If I set 15 days for Article Content Type, it needs to place "New" on all content of Articles that were publish in last 15 days.

Please, any suggestion will help.

Kind regards.

Hello,

Suppose I need to make only one content (in this case let's say it is an Article) different from other contents (the rest of Articles).

Specifically that content needs to have different CSS class so it will look bigger then others Articles when shown in list of teasers. 

Please any suggestion is welcomed,

Kind regards.

There is an entity form on the website. This entity form has a checkbox with the label Remind me about an event
If the checkbox is checked, I need to display two additional fields in this entity:

Drupal 8 and up don't like users to "do" PHP. That's somewhat understandable, although I feel the dreaded security risk is a tad exaggerated: a bit of basic user management can prevent a lot of harm. So, I for one Do like to have PHP available within Drupal, as it was in olden days - especially in a localhost development environment. Understandably, it was a very sad day when I found out that Twig templates don't accept PHP any more, like the PHP Template system used to do. I badly miss that!