Module development and code questions

I am upgrading a lot of inherited custom module projects to Drupal 8. In almost all of them, I am getting this error when scanning them with Drupal 9 upgrade status:

Class PHPUnit\Framework\TestCase not found.

I've Googled, and even found an accepted answer on StackOverflow.
But I am not able to figure out what to do.

My PHP version is 7.4.3.

I 've tried to install phpunit.

Hi, this is my first topic here and I hope have more clearly this doubt.

On D8 I could cast a field into a "orderBy" dynamic queries, but now in D9 I got an error, and I try to use other ways to CAST or CONVERT a field but I still got syntax errors...

I have this on my storage php file that is into a function I called from a Controller file:

$order_field is received from the Controller php file.

static function getSortedLimit($limit,$order_field) {...

Hello, 

So far I have used contributed modules for D7 & 8. I like to start learning D9 module development.

I am looking for some free courses just to get some basic knowledge before I purchase any paid course. 

Can you recommend me some guide, free and paid courses to learn D9 module development?

Thank you!

I have php block which displays links based on the combination of roles.

Hello,

Drupal chatbot module not compatible with Drupal 9? have others modules to replace it?

Do you have any suggestions?

Thanks in advance

My company is responsible for developing a vertical scrolling list of donors to a shrine, that will scroll via mouse click and drag. The list will be displayed on a touch screen interface, so that visitors can just drag the list up and down to view the names. We have the list created, however, we are unsure of how we can make it " scrollable ".