I don’t know how my WordPress got changed to a “standard” format but I cannot find my way around it.
Post format is a theme feature, and Standard is the default (and what would be used if no format or an invalid format is specified).
As we don’t know your website or even your theme, it’s impossible to say what may be going on.
i cannot find where to insert a custom HTML
Where do you want to insert this “custom HTML”?
In the page/post editor, use the aptly named Custom HTML block. If you’re using the classic editor plugin, just hit the “Text” tab and enter your custom HTML code along with the rest of the page’s content.
If you meant something else, kindly clarify exactly what you’re trying to accomplish.
Good luck!
