Typical freelancer marketplaces like Upwork or Fiverr give you access to an immense pool of freelancers and pay for them through these platforms.
In exchange, these platforms get paid a percentage of the payment to the freelancer.
Their job is to gather the largest group of freelancers possible, so you have more people to choose from.
Once hired, you will communicate and work with this freelancer directly, but there is no guarantee that they will deliver what they promised and meet your expectations.
At Growmodo, we only work with a vetted group of designers and developers that we contract on a full-time basis and manage your workload through a dedicated project manager.
Here is an overview of Growmodo and the typical alternatives: