There are basically two approaches:

• Social media agencies that manage company accounts on behalf of their clients and have their employees produce content for them.
• Agencies that operate their own accounts, which are financed through product placement, e-commerce (mostly dropshipping), or affiliate marketing.

Typically, these companies pursue both approaches simultaneously.

What they offer the actual content producers, i.e., the (sometimes even pseudo-self-employed) employees, is the following:

• A salary or at least project-based remuneration
• A network of contacts to advertising customers and thus lucrative sources of revenue that are pretty much unattainable for individuals without significant reach (they have sales people to protect those contracts from the people that do the content of course - usually these people are called account managers or something tacky along those lines)
• A network of contacts to other "influencers" in order to gain subscribers, etc. through strategic cooperation
• Know-how on how to build up accounts
• Professional equipment (cameras, dongles, drones, video editing applications and so on) as well as social media marketing tools for reporting, planning, and automation, which are not exactly cheap
• In some cases, substantial advertising budgets for ads to promote new accounts (performance marketing) and, in the case of campaigns for external clients, "seals of approval" from meta and other Plattforms (meta, Google or TikTok "Business Partner" for example — these seals are exclusively issued to companies who spend a significant amount on ads on the respective platform)
• Opportunities to collaborate with other employees of the company, which can also create network effects.

There are certainly other advantages, but the key point is the contact with advertising customers, i.e., companies that want to engage in social media marketing. These contacts are only accessible to private individuals if they already have one or multiple successful accounts, which unfortunately only very few of those aspiring to a professional career in this field ever achieve.