Your confusion makes sense, this is because WordPress is so easy and automated out-of-the-box it’s a whole new way of looking at it when getting into customization.
You clearly installed Startup automatically directly from the theme search under Appearance > Themes. There’s also a semi-manual and manual way to accomplish the same thing, but with more control.
Semi-manual you could go directly to https://wordpress.org/themes/startup and download the zip, then upload and install under Appearance > Themes.
The fully manual way (which is what we need to look at for child theming) is download the zip from https://wordpress.org/themes/startup, then unzip it so we just have the /startup/ folder and then manually upload it to /wp-content/themes/ via FTP (File Transfer Protocol).
What host are you on? Different hosts have different requirements/instructions for connecting via FTP and they should be able to guide you on this.
What we need to do now is get the /startup-child/ theme folder with your alterations uploaded to the /themes/ folder and then activated under Appearance > Themes.
I realize this might feel a bit overwhelming at the moment, but you’ll be zipping along in no time. :)