first I'd like to thank you for choosing the right section of the forum to post your questions
It's not possible to place a HTML SNippet into a TextBox. The HTML Snippet will always float around on its own. Usually it's no problem selecting the HTML Snippet box to move it around. Just click on it, release click and move it around. If that doesn't work you could try selecting an entire area (multiple objects at a time) by click (and hold click) and move the cursor around. Once you selected the area and objects you can deselect the ones you don't need by hitting Command(Apple)+click on them.
It's not possible to attach a Snippet to a paragraph so that it moves with the text when the text gets longer. That's why I don't use HTML Snippets and use what I call Alternative HTML instead (see Alternative HTML FAQ). This way it moves with paragraphs.
About YouTube... If you want to embed the videos into your page you can add several "YouTube Embed-Codes" into one HTML Snippet. Have a look at my YouTube video FAQ if you want to have a menu from which people choose a video and different videos displaying in one area.
You don't need an HTML Snippet to add a URL to a page.
Simply write the text you wand (example: Have a look at this video!) select the text open the Inspector go to the link tab (right end of the Inspector window) and check the "Enable as a Hyperlink" box. Link to => An external page. URL paste your URL. And check the "Open link in new window" box if you want it to open in a new window.