The principal difference between them is that SEM will keep ads visible as long as they are paid for and remain ahead in the auction for space on pages (paying more or with higher quality ads, keywords, etc.). This is considered a marketing expense because the day that payment ceases, the ad will immediately cease to show up. SEO, unlike SEM is more of an investment. Everything that is invested has the capacity to make returns in the medium-long term, with the SEO “organically” making positive returns on campaigns.
If someone needs it, someone will sell it. If there is even a minor demand in the market for your product or service and you are not selling, this means that at some point in the process of reaching your customer, you have failed.