You can use https://www.wireshark.org to see HTTP traffic and MITM yourself for HTTPS(https://unit42.paloaltonetworks.com/wireshark-tutorial-decry...). That will only work for the data that you send to them yourself. You can't possibly see any data that they send to third parties that they send through their servers.

Disconnect browser extension had a graph which domains are requested, is it that you remember? https://commons.wikimedia.org/wiki/File:Screengrab_of_%27Dis...

There are lots of ways to do that. You can look at the developer tools provided by your browser. You can also run your browser through an intercepting proxy like burp or zap. The latter gives you the ability to intercept and step over actions, so you can really slow things down. I'd recommend using on of them.

The manual option is using ghostery and clicking in to each pixel URL to look at the paramaters. Many times the data is not obfuscated (in a facebook pixel you can see things like val=Ford, when you're browsing a page for a Ford truck)