Find me which API some company uses

This project is for someone who is really intelligent tech guy and know to work out of the box.

I know few website that uses third party information and shows them on their website. I need to know who those third parties are.

Below are the companies that I need information on.

1) Signalhire - They sell web extension that fetch contact information on the linkedin. It's obvious that these data comes from the third party. I need information where it comes from?
2) Swordfish.ai - They sell web extension that fetch contact information on the linkedin. It's obvious that these data comes from the third party. I need information where it comes from?
3) Kendoemailapp - They sell web extension that fetch contact information on the linkedin. It's obvious that these data comes from the third party. I need information where it comes from?

Either they are using API to third party or some other technology to show data on their web extension.

I just need this information. To initiate a chat, send me some credible info so I can talk.