Include assignee in issue GET requests, also assign assignee via API?

We’re trying to set up integration between a security product and our Jira Service Management cloud instance. Before trying to set fields in our third party app, we’re just trying to use PostMan to see what format these should be in and are having trouble. I’ll be the first to admit I have very little API/REST/PostMan knowledge.

We have to use basic authentication for our POC and we can get basic queries to work, but assignee is ommitted. When trying to get assignee, it doesn’t seem to like any type of query we make. Since we can authenticate and get some information, we assume it isn’t an auth issue (The api token user has full project rights) and are assuming we’re missing how to set up the get/post/put statements correctly.

tl;dr - Does anyone have an example they can share of:
How to query via rest api for full issue details (or at least just a basic response + Assignee)
How to set via rest api an assignee for an issue on create And/OR edit an existing issue to set an assignee via api

Please let me know what details would be helpful and I’ll respond as soon as I can, thanks in advance!

Pages I’ve referenced so far include these and more but I think these are the most valid?