Remove ‘We’re happy to have you here’ text

“Welcome to our community
We’re happy to have you here. If you need help, please search before you post.” I would also like to know how to delete the text that appears at the top.

Those are strings of the search banner. You can find the settings for that theme-component at YOURSITE/admin/customize/components.
Then you can choose the search banner and edit the texts in the Theme Translations.

All in all it is more helpful when you ask different questions at different topics. Then it is easier to find them while searching. Furthermore you can use a meaningful title and select a solution for every question.