I actually got this same question internal from support so hopefully you got the answers you need, but I will drop an answer here as well for future people asking something similar.
So in general we have 2 different API
XML/RPC (Legacy)
REST (Preferred and where all current development is happening)
Please check out our developer site https://developer.infusionsoft.com to read through the docs to get answers to answers to your questions. The docs have the list of endpoints, which HTTP Methods to use, etc. Also read through the Getting Started guide which will show you how to authenticate, some thing to be aware of like throttling, etc.
If you have any questions after reading through the docs and Getting Started Guide, drop a question here in the Community and there are many helpful members (as well as some Keap engineers).