When you add a plug-in, it just becomes available for anyone to use. Ditto for media. In general, we don’t have to use a plug-in or attachment you bring onto the site. However, I think if you installed and activated one of those “site-builders”, like Elementor or Beaver Builder, that might affect everyone. I suggest that for now you see what you can do with WordPress “core”, without that type of plug-in.
Note added later: In fact, BoldGrid is one of those “site-builders”, although I’ve deactivated many of its plug-ins. Still there is the Post and Page Builder. At the moment I’m not using it; I much prefer the WordPress editor. You can read about it at https://www.boldgrid.com/support/page-builder/.
If you use a plug-in to make a block in the footer or the sidebar then, of course, that block will appear in the footer on every page and in the sidebar on the pages that use the sidebar. (At the moment, only the posts pages have sidebars.) If you find a plug-in you like, let us know about it after you have tried it out. Many are not as useful as you might have thought but there are thousands out there. Don’t pay for any of them at the moment; try out the free version and be sure there isn’t a free one that does more. The plug-in business is very very commercialized.
Another added note: You need only to look at boldgrid.com for an example, or the JetPack site for another. JetPack is another plug-in that dreamhost.com installs for free (although not, unlike BoldGrid, the premium version.) There are many excellent plugins that aren’t just teasers to get you to buy the premium version.
So you are not working on your “own” website. Everyone can see everything you do, assuming of course that you don’t hide it away somehow so that it doesn’t show up in the Pages or Posts displays. (Actually, I think nothing is hidden from an administrator, and very little from an editor.)
If you create a new page, that’s available for any other administrator or editor to view or edit, but I would suggest that we not do that. If you look at the “authors” at the moment, there are some pages of which I am the author (“Kathy”), but most pages are ascribed to “5CLIR”, which means they came from 5clir.org (although if you look under Users, you will discover that the screen name “5CLIR” actually is associated with another e-mail of mine! I will fix this some day so that it is associated with the office e-mail, which I should have done in the first place.) Pages that you create will be associated with “Tyll” or “Bob” [You can change your screen names under Users. You can also change your password, you are not stuck with the one I made up. And you might like a different Dashboard color scheme; you can fix that too. All of that is your choice.] If you see improvements that are needed on the 5CLIR pages that’s fine with me. I’m working through them slowly because they are pretty awful, with random type faces, broken links and missing attachments. More specifically, the links and attachments take you back to 5clir.org rather than staying on this site.
So I am suggesting that before you start trying to make basic changes in appearance, you create some pages and file them under Demo Pages => TVG Demo Pages or Demo Pages => RC Demo Pages. Incidentally, those are the first two pages of which you are respectively the authors! You can try out anything on one of your pages without affecting the rest of the site. Try some of the block types, some of the plug-ins, …
And also make some posts, assigning them to the category Web Site Notes. They will show up on the Blog page. If you fill out the “excerpt” box over in the right column, then that’s the text that will be reproduced on the Blog page; otherwise WordPress will try to extract something. (I recommend filling out the excerpt box!) If you do something particularly nifty, tell us to go look at the page you created (and provide the link; learn how to add links.) There is a way to list blogs by author in the sidebar, I’ll try to implement that. That way by clicking on “Tyll” you should get to a page that shows just your posts. Nothing private here! I can also click on “Tyll”.
Send me questions, “How do I…?” I’m going to start a FAQ page (it will be under KC Demo Pages.) It will include a lot of things I continually forget too.
Where you will start affecting the whole site is when you start customizing the theme. That changes the appearance of every page. I have done minimal customization so far. So when you are ready to start, be my guest. And take notes! because I haven’t figured out most of it yet either. I kind of think that if you activate one of the “templates” that will affect every page. But maybe not. Really a template should only affect the peripheral parts of the page: the header, footer and sidebar. Templates should not be messing significantly with the content, a.k.a. the Primary Container. (Read about “containers” at https://wpastra.com/docs/container-overview/. The container type I have selected is called “Full-width contained”. Feel free to try the others to see what happens, but I think you’ll come back to this one.)
Incidentally, my experience is that when I customize something, it can take a few minutes to show up. Use the “Delete cache” button at the top of the dashboard; that might fix the problem. It doesn’t hurt anything, just maybe makes pages load more slowly for a while; you won’t notice.