Hello,
Thank you for contacting Rank Math today.
Rank Math plugin adds the necessary markup for rich snippets. It may take some time before they show up in the search results, please allow a few days for Google to pick up the information.
Google will not necessarily show the FAQ Schema, though, they check a few things first and may choose not to show, for instance, they check the overall quality of the site, and if the FAQ content is representative of the main content of the page or not.
Here is a quote from one of the Google’s help article:
We perform algorithmic and manual quality checks to ensure that structured data meets relevancy standards. In cases where we see structured data that does not comply with these standards, we reserve the right to disable rich snippets for a site in order to maintain a high-quality search experience for our users. Read our webmaster guidelines for more details.
Link:
https://developers.google.com/search/docs/guides/intro-structured-data#structured-data-guidelines
If you believe that the rich snippets have not been added to your site. Send us a link to that particular page you are testing and we can check it using this tool:
https://search.google.com/structured-data/testing-tool
About the description:
1. First of all, I am assuming that you have already changed the SEO Meta Title and Meta Description with the help of Rank Math:
Add a Meta Description in Classic Editor:

Add title and description in Gutenberg:

2. Then, ensure that this is the setting in the Schema tab if Rich Snippets are enabled on your website:

To reiterate, the Schema title must show %seo_title% and the description should show %seo_description% – this will ensure your SEO title and SEO Description that you set up via Rank Math can also be used for your schema details:
3. The next step is to check if your title/description is properly set up in the page source:
https://i.rankmath.com/HwXR1o
You can use this tool for the same as well: https://www.heymeta.com/
4. If it matches your settings, then you must check if Google has seen the changes already or not.
For that, please check when the Google cache was updated for that page:
a.

b.

If the cache date is from before adding the new meta description, then you just have to wait for Google to re-crawl and re-index the page with the new info. If the date is after you made the changes,
Do note that if everything’s fine and Google still decides to show a different meta title/description for your search keyword, there is nothing you can do as Google sometimes ignores the custom meta info altogether and show something from the page’s content that matches the search intent better.
The best you can do is optimize your meta tags to try and match the intent of the search/keyword.
You can read more about it here
https://support.google.com/webmasters/answer/35624?hl=en
This section sheds light on that:
Why the search result title might differ from the page's <title> tag
Hope that helps. If you have any further question(s), please let us know. Thank you.