About code scanning
You can use code scanning to find security vulnerabilities and errors in the code for your project on GitHub.
About third-party code scanning tools
Code scanning is interoperable with third-party code scanning tools that output Static Analysis Results Interchange Format (SARIF) data. SARIF is an open standard. For more information,

You should dismiss a code scanning alert if the flagged code is not a true security concern, such as:
*-> Code in test files
Code paths that are unreachable or safe by design
False positives from the scanner
Fixing the code would automatically resolve the alert - not dismiss it. Dismissing is for valid exceptions or noise reduction.

To receive Dependabot alert notifications for a repository, you can utilize the following Watch settings:
Custom setting: Allows you to tailor your notifications, enabling you to subscribe specifically to security alerts, including those from Dependabot.
All Activity setting: Subscribes you to all notifications for the repository, encompassing issues, pull requests, and security alerts like those from Dependabot.
The Participating and @mentions setting limits notifications to conversations you're directly involved in or mentioned, which may not include security alerts. The Ignore setting unsubscribes you from all notifications, including critical security alerts.

The options available to specify the additional queries you want to run are:
packs to install one or more CodeQL query packs and run the default query suite or queries for those packs.
queries to specify a single .ql file, a directory containing multiple .ql files, a .qls query suite definition file, or any combination.