Once you pick out a theme, install it, and create a child theme, you have all the basics in place.
Then in the theme ( not the child theme ) find the php file used to display the posts. Lets say it is called blog.php. Make a copy of that file and place it in the child theme, then WordPress will take note of the file that is using the same filename and use that one instead of the file in the main theme.
You then would rewrite the code that displays a blog post to suit your requirements. And yes, you would use the “Add New Post” in the WordPress backend to create your posts using whatever editor you choose, lets say the “Classic Editor”.
To style the page ( or any page / post ) you add your css to a file in the child theme called style.css
If you decide to write a custom template for a page other than your blog posts you would probably not copy some file from the main theme to the child theme, but rather create one from scratch. That is not always the case, sometimes the template that comes with the theme is pretty close to what you want and you might “borrow” that code to use in your custom template. By the time you are doing any of this it will all make sense because if you are anything like I was at this stage I was reading “how-to” articles all of the time on everything.
My advice, set up WordPress on a server, find a theme, install it, create a child theme, and then pick away at it from there. You will answer a lot of your own questions just poking around and trying different things.
About ftp. Most decent IDE’s support two modes. If the website is installed on the same computer as the IDE then there is no need to use ftp, the IDE justs opens and edits the files directly. The other mode is a remote mode where the website files are on some other computer/server. In this setup the IDE connects to the server via ftp and then for most practical purposes behaves just as if the files were local to your computer. You open the file, make your edits, then upload the edited file to the remote server.
Do a search here on FCC for IDEs, there is a ton of great advice. I purchase mine ( PhpStorm ) to get all the features I need, but there are a lot of really excellent free ones out there.