First client contract - Need Help with Real Estate with RETs

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How do I integrate RETs mls listings into my solution?

I'm doing a contract for a Real-Estate company and basically have to integrate a solution to list all MLS real estate listings on a page. There are other specs as well but they aren't an issue. I'm building with RoR (with pg) and deploying with AWS elastic beanstalk (EC2).

Initially I thought I was going to use a simple API endpoint which had some json data but this isn't available in the Real Estate industry. Instead I had to access the IDX solutions through which I have to use RETs, a database for MLS listings. However I'm so confused as to how I should integrate RETs and what it exactly is?

If there are any Devs with Real Estate experience, can you please help me understand how to integrate this?

I'm currently looking at alternatives like https://simplyrets.com/

Thanks in Advance.