Processing returned HTTP REST data

I've used Power Automate (actually it was written in Microsoft Flow) for a few things, but I think I need some help on how to handle HTTP requests.

tl;dr: How to I 'extract' variables from a returned HTTP response?  (Got back a JSON object, how do I parse for a single variable to store?)

My overall process looks like this:

• Trigger: Item added/edited in SharePoint List <-- Got this
• Filter for things that have already processed <-- Got this
• Take one of the fields and make a REST call to a web property (non-Microsoft) <-- Got this
• Get back a JSON blob containing a bunch of information, but specifically, I want a single entry (call it 'UserID') <-- Lost here
• Store UserID in a variable. <-- Pretty sure I got this part
• Use UserID to perform another REST call to the same web property <-- Still lost
• Extract information from that returned JSON blob and update the SharePoint list item. <-- Pretty sure I got this part.

If there's any resource that would be helpful, just point me in the right direction.

Note: I can do the REST stuff in PowerShell using the `Invoke-RestMethod` function, but I don't want to rely on someone running a script to do this if at all possible.