Many probably most monetarily wealthy people have multiple sources of income. This is true. They might work a 9-5, have a side hustle, and invest for example. At the end of the day, you want money you’ve earned to earn you money. Understand maximizing income will often be detrimental to other parts of your life. Time is a finite resource. It’s difficult to work at a high level, run a successful side hustle, and do the necessary research to invest intelligently. Throw in spending quality time with your girlfriend (in your case) or your family, exercise, and any other activities you enjoy and you’re bound to run into difficulties.
Wealth isn’t just measured in $$'s. The sooner you learn that the happier you will be.
At your age, the best thing you can do is sit down and map out the pros and cons of professions you’re interested in and then make a plan to enter the one you prefer the most.
When you enter that profession immediately contribute to your 401(k) at least enough to get a company match if it’s offered. Your goal should be to ultimately max out your 401(k).
IMO, you should also pay yourself x% of your income as an emergency fund. Some people suggest 6 months - years salary. I don’t think that’s necessary, but it certainly won’t hurt. I’ve found the easiest way to achieve this is to open a separate bank (even at a separate bank) that you can’t easily access and have money direct deposited from each paycheck.
For example, I have a Capital 360 account that I only transfer money in and out of. It takes several days to move money around so if I see a shiny new toy that “I just have to have” I’m forced to spend several days thinking it over. In other words, it cuts down on impulse buying and make savings effortless.
Then invest in an IRA or a side hustle or both if you want additional income streams, but IMO the foundation for the vast majority of people should be a well-paying career with growth potential, maximizing 401(k) contributions, emergency fund, and then x, y, and z alternative income streams.