You can buy solar panels from a professional solar company, from a dealer or do the installation yourself
You can compare buying solar panels as installing a new electrical service panel in your house. Some people can do that electrical work themselves. They know the work, they know how to stay safe and they know where to buy the hardware. Most people don't want to take on a project like this and want a professional to take care of it.
You can buy solar panels from different companies. You have the vertically integrated solar company that does most of the work in-house ranging from sales to design, permitting and the installation itself. There are also "dealers" or middleman that offer you advice or "the best price". These companies do the sale and rely on others to do the installation. Can you read more about who's who in the solar industry here: https://knowledge.ipsunsolar.com/who-can-i-trust-in-the-solar-industry-whos-who
Just remember the saying "You get what you pay for."