So I just got Sims 4 good to go and played a hefty 10 hour session, making some good progress. Question about the library thing, having played Sims 3 for hundreds of hours:

Say I want to make a new playthrough with a new save/sim but I want to keep the previous one as a townie. Do the previous sim's progress (aspirations progress, traits gained, skill levels gain, money, etc.) carry over to the new save like Sims 3? Or does the game only save the CAS of that character or saved as a preset sim and I have to start all over again from scratch the moment I switch back to that character?

Because if it only saves as a preset sim in the library, that'll be extremely disappointing.

As far as I remember traits (including reward ones), skills, aspirations and careers progress will carry over
but all relationships outside of the immediate household, inventory items and perhaps household founds as well - won't

Yeah I understand the relationships and items. All I needed was the skill progress and aspiration trait progress. I find it somewhat trickier than Sims 3 which was just straightforward. Especially the Lifetime points or the Satisfaction points don't regenerate like in Sims 3.

And while we're at it, can you give me a step by step how to save the existing household so that I can carry it over? I'm still new to Sims 4 UI, it's still hella confusing.

If you take the family into CAS you will see an option in the right upper area that looks kinda like a folder. Click that and the family will be saved to your library.