You’d save it to the database in the same field as the rest of the text. You don’t store the positions or anything like that - you’d store the text with HTML and have the front end render it as expected.
For instance, the database could have the following text:
Hello <strong>World</strong>
And the front end just renders HTML.
Alternatively, you could store Markdown syntax if you’re hesitant to allow HTML.
EDIT: as always, if you store raw HTML, don’t forget to sanitize it.
Of course not lol. The CMS I usually use stores it as HTML in the database, so I have a go-to HTML sanitization plugin with a tag whitelist. I wish it used markdown or something similar under the hood instead, but it is what it is.