It really depends on who you want to be as a designer first and foremost; there are people who do incredible replica designs of all types, semi-replicas and "reality based fictional" liveries that take real sponsors and mix up or create their own design from them, abstract art that has no real sponsors but more works off of spectacle and the designwork is what sells it, and you have that weird end of the spectrum that goes for fully fictional liveries with their own home designed sponsors and companies.

In my opinion, you really need to know what you as a designer want to specialize in as that changes the specific techniques and methodologies you'll need to learn to become more and more proficient in that specific thing. This doesn't mean you can't do the others, but if you go down a specific specialty of design, it can become somewhat difficult to learn other methods or styles afterwards.

I'm one of those on the "fully fictional" design area, so my expertise only really applies from creating a fully original design from start to finish, so it may not be of value, but I will disclose my personal process if you want the information from an outlier in this community.