From what I understand using a child theme is important if I don't
want to loose anything when updating the theme. I am a beginner at
WordPress and so far I have been customizing my site using
PageBuilder, the onboard customize option on the admin panel and
putting in a few lines of CSS in the "additional CSS" option.
None of these changes will be affected by a theme update. The reason changes to themes will be lost when the theme is updated is because when a theme is updated all the files in the theme folder are deleted and replaced with new versions. So if you have modified any of these theme files then those changes will be lost because the file has been replaced.
The types of changes you're referring to are using settings provided by the theme and saved in the database, so they will not be affected if the theme files are replaced.
The theme being a parent theme is not actually relevant here. That's a common misconception. What is relevant is whether or not the theme can be updated, because it's the update process that breaks things. If you download a 3rd-party child theme and modify its files, those changes will also be lost if the theme updated, regardless of whether or not it's a child theme.
It's actually the act of create a brand new theme (child or not) that means your changes can't be lost to an update. This is because no one else will release an update to your theme. Being a child theme just means that you get this benefit while getting to keep all the styles and functionality of another theme.
I don't understand if I can design my child theme the exact way I did
with the parent theme through the admin interface or if I have to code
everything separately through text editors (on my computer and then
upload theese through for example an FTP).
Can I still use PageBuilder and all of theese extra CSS lines I
It ultimately depends on how the original parent theme was built, but usually a child theme will work exactly the same as the original, so you'll still be able to use the page builder.
Lines of CSS saved to Additional CSS are saved per-theme, so when you switch to the child theme you'll need to copy and paste any CSS from the Additional CSS section.
Will changes I make to the child theme through the admin panel add
code automatically to my child theme, or will most of the changes
still be made on the parent theme?
When you activate the child theme you should be able to make the same types of changes that you made to the parent theme, but when you switch themes the settings will reset and you'll probably need to make them again for the child theme.
I say "probably" because it's theoretically possible that the original theme saves these settings in a way that they won't reset. The specifics depend on the theme, and is something you should ask its author about.
I've read around a bit and I just can't figure these things out. It
feels like if you are using a parent theme, you can edit everything
through the admin panel and so forth, but when you use a child theme
you have to code it on your computer and then upload it all..
Whether a theme is edited in an admin panel or by code has nothing to do with whether or not the theme is a parent or child. The point of a child theme is that if you want to make code changes to the theme, you should do it through a child theme.
Making code changes to a theme that isn't yours isn't safe, because updates can erase the changes, so you should use either a child theme or create a new theme from scratch.
Based on everything you've said though, you do not need a child theme. The types of changes you are making are safe from updates.