Fallout 4 noob here. I just picked up GOTY in the Steam Summer Sale for my iMac
Companions-
The games has a long list of companions who all want to adventure with you.

You get a skill from each one when they reach their highest admiration of you, which is not difficult, just spend time with them, however they have moral leanings. If you romance them, you will get a buff from sleeping with them.
Regarding moral leanings, for example Piper does not like it if you mistreat humans, while Cait does not like Synths or if you help them. And depending on the companion you can lose points with them, but overall it’s nothing to worry about, the upward trend of affection points by traveling and fighting together will outweigh any downward blips based on moral choices, however I do keep these prejudices in mind and sometimes choose companions for specific tasks, but it’s not something to be concerned about.
The base game offers 13 unique permanent companions, each with their own quirks and storyline. Add-ons introduce four additional companions; Automatron adds two fully customizable robot companions; Far Harbor and Nuka-World introduce a single, narrative-focused companion each. With all add-ons...
fallout.fandom.com
A guide to companion likes and dislikes in Fallout 4 so that you can get their unique perks.
www.carls-fallout-4-guide.com
In addition all companions have ID codes and instead going to fetch them, via the console (PC) you can summon them to you. And if you are worried about possible hard feelings when you dismiss a companion, or romance multiple companions, it’s not an issue, they are most understanding and are happy with the time they get to spend with you.
The mechanics of companion exchange is if you have a companion with you, you ask the new companion to travel with you, a screen pops up and asks which settlement you’d like to send the old companion to.
Of note for someone like Nick Valentine who lives in Diamond City, Diamond City will not be listed as a choosable settlement. For both him and Piper, if you want them to return to DC, do not choose a settlement, just close that window and by default they will return to the last place they were based, which for both of them is Diamond City. The only caveat for both of these companions, is that if you do send them to a settlement, after that, there will be no way to send them back to DC without the help of a mod.