Policies vary. Where I work, you can seek permissions to work an outside gig that is "regular or continuing". The request might very well be met with a firm "Uh, yeah, no, nope. Not signing off on that." I actually don't know, I don't know anyone whose made the request.
On the other hand, the requirement to ask permission only applies to "regular and continuing* work. If it isn't "regular" (say, temporary contractor gigs) then there is no obligation to report or request permissions. Verbiage on the issue says the employee is responsible for making a reasonable determination on whether or not it's regular or irregular.
So, rules vary. Intel for example has baked into their employee code of conduct the assumption that some people will have outside employment &/or businesses, and simply provides a detailed code of conduct around how to navigate any issues that might arise from it: https://www.intel.com/content/dam/www/public/us/en/documents...
The tech world especially is in the era of the "side hustle". Taking on an actual extra full time job under the premise that a person can do both in under the nominally required 40-hour work week probably (well hell, all but definitely) pushed the limits a bit over the line, but some type of outside employment is often not impossible & well within the guidelines of many reasonable companies.