How do I know if my theme is a child version? To my understanding a child theme I am able to keep all of my own CSS on it, but if I have a regular one it will not keep my CSS on it if I do a Sydney update, correct? Is there a way to have both or what is the best way to go about this?
You can read more about child themes on WordPress Codex https://codex.wordpress.org/Child_Themes and there is enough info in case that you want to create your own child theme.
In case that you just want to apply your css code and nothing else then you don’t have to use child theme since you have custom css field in WordPress: Appearance > Customize > Additional CSS field and also there are many plugins for this which will store and apply your css code.
How do I find out if I have a child theme or a parent? I honestly do not
know. I never created one, so I am assuming it is a parent one. I do
occasionally use the custom ccs field, so if I want to use that a parent
theme should be just fine when I go to update the theme, correct?
If you see Sydney theme with its image like shown on WordPress org https://wordpress.org/themes/sydney/ then you have parent theme, and yes, it is fine to use it with appearance custom css field since your code will be saved in WordPress database and it will be available even if you update or change theme.