Confused about difference between a locked file & a read-only file

I have a Mac & my sister has a Windows computer.

I use Libre Office if that helps. She has Word, I guess.

I sent my sister a document in .docx format. She made changes to it without my permission & sent it back to me. I spent over an hour changing the document back.

I don't want that to happen in the future.

I'm confused about the difference between a locked document & a read-only document.

I haven't been able to find much information online. I believe that a locked document can't be deleted.

I only want her to be able to read the document but not edit it. When I clicked on Get Info for the document that I sent, it shows that I have read & write privileges but Everyone has read-only privileges. For some reason, I also saw Staff which I had never seen before.

When I opened the document, there was a blue bar at the top stating that the document is in read-only. All I had to do was click on the button on the right-side of the blue bar to edit it. I don't want my sister to have the option of clicking on the button on the blue bar.

What will happen if she tries to edit a read-only document? What message will pop up?

Do I need to just set the document to read-only? Do I really need to lock it?

How does the document that I created "know" that is opened on a computer other than mine?