Managing Contacts without linking any accounts A friend gave me her Samsung Galaxy S7 when she upgraded her phone.  We ported my number onto her account, and now I (a long-time flip-phone user) am trying to figure out this new device.  

Important context: I do not intend to sync up my gmail account to the phone.  I am combating an internet addiction by refusing to have the internet in my pocket constantly.  Plus, I find it creepy when devices "talk" to each other; it makes me feel powerless and confused if what I do in one setting has effects in another, unrelated setting.  In other words, I want the improved reception and the threaded text messaging of the new phone, but otherwise want it to be as "dumb" as possible for my peace of mind.  

I spent all last night manually adding contacts to the device.  The whole time, my friend's gmail was still synced up, and her emails kept popping into my view.  So this morning, she removed her linked accounts.  As soon as she did, all the contacts I had manually entered the night before disappeared, the contacts I had edited the night before reverted back, and a bunch of them became "Read Only" contacts!  

This presents several problems/questions:


Did the contacts I manually entered go into HER gmail account?  She doesn't need all my personal contacts!  
Do I have to manually re-enter them, and if so, how can I be sure that they are being saved to my SIM card?  (In case I ever get a new device, it would be nice to simply move the SIM card and keep the contacts.)
The "read only" contacts are a good mix of ones I want to delete and ones I need to edit (fix misspelled names, add new numbers).  How do I manage them?  If she un-linked her accounts, why are these "read-only things" still there?  


Again, I am looking for solutions that involve only my phone and do not involve linking external accounts!  It would be super weird if all my gmail contacts appeared in my phone.  Phones are for phone stuff and email is for email stuff, and having them bleed into each other seems like poor management.  

Incidentally, there is now a message on my phone telling me that my data is not backed up.  This confuses me, because while I thought "data" meant "bits of information", all my friends corrected me that nowadays "data" means internet usage, and I should just be able to "turn data off" so my phone isn't constantly online.  But obviously it means "bits of information" if it is something that can be backed up.
The locations mentioned below where you can find settings may vary across Android versions.


  Did the contacts I manually entered go into HER gmail account? She doesn't need all my personal contacts!


Where do contacts go can be selected with Contacts -> Three dots menu -> Manage contacts -> Default storage location.

Among other options, you have Samsung account (if set), Phone, SIM card, Google (if set), Outlook (if set).
This is exactly what I see in my phone (I have actually three Google accounts).

I guess (did not try it) that if the default storage location is a Google account, and it is removed, the default changes automatically.
So your current setting would be probably not indicative of what it was before.
The best way to check that is entering her gmail account in a web browser, and see if the contacts you added are there.


  Do I have to manually re-enter them, 


If they went to her Google account, you have several options to avoid manually re-entering them. Depending on the number of such contacts, the economics equation on what is most convenient may vary.

You can take advantage of Contacts -> Three dots menu -> Manage contacts -> Import/Export contacts.
I have used that to transfer csv-formatted contacts list from an iPhone to Samsung, avoiding any additional app.


  and if so, how can I be sure that they are being saved to my SIM card? (In case I ever get a new device, it would be nice to simply move the SIM card and keep the contacts.)


See above.

Plus #1: 
You can use Contacts -> Three dots menu -> Settings -> Contacts to show.
You can select any contacts source to be used when showing contacts. You can even select a Custom combined list.
This is what have.
Then, whenever you change the source, you would be showing/hiding contacts, and that can tell you the location of each one.

Plus #2: 
Whenever I Edit a contact, at the top of the screen I see the source of that Contact (Phone, Google, Skype, WhatsApp, etc.)
You should have to tell first the appropriate contact display list as in Plus #1.


  The "read only" contacts are a good mix of ones I want to delete and ones I need to edit (fix misspelled names, add new numbers). How do I manage them? If she un-linked her accounts, why are these "read-only things" still there?


Not sure. Never faced this, but I found a variety of culprit sources of Read-only contacts out there, and correspondingly different solutions.
E.g.
https://www.quora.com/How-do-I-delete-read-only-contacts-from-my-Moto-G2