What are some learning resources I can use to get there? How did you guys build something in a day?
Do you want to ship something or learn a framework ? Nothing wrong with either, but you must know the answer. Nothing is wring with learning a framework by building something either, but here you're using the product to learn a framework/stack, and not using the framework/stack to build the product. Grosso modo: don't lie to yourself about what you're doing.
Strip everything that does not invalidate your idea. Spend zero time on password reset flows or automated emails. Do things manually. Just get the email address and get into a conversation.
Doing more projects to gain skills so building the prototype is not the bottleneck because you're efficient. Do things better.
Doing fewer projects by validating with the minimal thing you can ship so building the prototype is not the bottleneck because you're effective. Do better things. Validate beforehand. Talk with people earlier. All these don't require you to build things.
Basically, stick to your core competencies, but don't be afraid to try new things that will clearly (hopefully) improve your product. Fail hard and fast instead of dragging it out 6 months :)
This isn't about learning a framework at the expense of the product, it's about building something new that you might not have experience creating solutions for yet.
1) A frontend: pick a framework/lib (or none) that works for you. Whatever lets you easily build the layouts and interactivity you're happy with.
2) An API: connect a server to your frontend (REST, GraphQL, websockets, etc.). This is where all the secure communication happens.
3) A database: pick a DBMS (usually SQLite or Postgres) and hook it up to your API. This is where you organize data to be displayed on your frontend, or run through business logic on the backend before displaying on the frontend.
Learn how to setup a project with all three quickly and efficiently, then you can build whatever you want. More complex stuff just means adding onto this stack.
As for choosing the stack for all of this, do searches like "what's the best frontend framework", "go vs node for backend", etc. and read everything with a pinch of salt - after some reading it should become clear what everyone thinks about all the various tech that's available. After you think you have some idea, just try it.
Did you follow some book/learning resource recently that you found valuable that you would recommend on this topic?
If you need a suggestion - Django with Postgres on Heroku is consistently the top prototyping stack of choice when it comes to Y Combinator companies.