How To Edit WordPress Themes and Code

Just joined Digital Ocean and I want to install Ubuntu and follow this tutorial: https://www.digitalocean.com/community/tutorials/how-to-set-up-multiple-wordpress-sites-on-a-single-ubuntu-vps (which I will use the newest version)

My question is: 1. After installing everything and getting WordPress set up, can I access the multiple websites and the Ubuntu server from any computer or do I have to work on the computer that I installed Ubuntu and WordPress?

2. Once I have WordPress installed and am working on a theme, do I have to edit the coding files from my computer (where I installed WordPress) and then upload them via FileZilla (or other), or can I edit the WordPress code files after uploading? I just want to know if I can use any computer.

I found this from https://kinsta.com/blog/how-to-customize-wordpress-theme/:

“If you’re editing your own theme, which is specific to your site and was developed especially for it, then you can make edits to the theme directly. But if you’re working with a third-party theme and want to edit that, you should create a child theme to avoid losing your changes next time you update the theme.” ** 3.** What does it mean by ‘editing your own theme’?

Sorry, many questions.