Re: Newbie question about saving Sims
The library is yours and the gallery is where others share their creations with other players. Your Households are where they are always available, whether in the library or not. Other Households is for Townies/NPCs. But there is a recommendation for the maximum size of your Households and you may choose to move Sims over to Other Households to make room by using the options on each household image in Manage Households. I do this often. It won't affect the Library copy. It means they become Townies and keep their skills and relationships. Other Households can be subject to culls where Sims are deleted but I regularly delete random Sims added by the game and keep the number below 100. I've not lost anyone I've created out of Other Households.
When you create your sim you will see the option to save them to your library if you want to. I don't always do that. It means that if you lose your Sim their basic self will be saved but any life experience aren't added to the library copy, unless you save them there again later. Sometimes that adds their skills, etc but not always and family links in a household may need to be re established.
So as an example. I lost my entire game but I had library copies of many of my Sims. I'd saved their full and completed households to my library and I still had the basic original Sims too so I had various options to add them back to my world. I had saved their houses to my library too. All I had to do was link them to their family members in CAS again, place their houses again and put them back. By using the households I'd saved with all their skills, etc. It meant they were good to go. If I'd added the original Sim I'd saved at creation they'd have had no skills, etc.
I then had a problem with my library and had to clear it all, so when I lost my game again last year I had to create all my Sims again from scratch.
I haven't saved to the gallery to share with others, I've been planning to but not done it yet. But I have downloaded others creations.
I'm on Xbox Series S and when I've switched to a new Xbox and logged in with my username I've been able to access anything I'd saved to my library.
Hope this helps!