There are lot of videos/tutorials out there on youtube, udemy etc but let me summarize what is needed to build a SAAS Web App assuming you don't know anything about web apps.

1. Fundamentals of Web including protocols (http/https), GET/POST requests etc.

2. Knowledge of HTML/CSS/Javascript to build based websites/web pages.

3. Learning a server side language such as PHP, Python, JS/NodeJS, Go etc.

3. Databases and how they work with web apps. Learn SQL and database design as well. How would you create Single Tenant vs Multi Tenant apps etc.

4. Basic Server knowledge to host web apps. I highly recommend learning Linux Fundamentals. Also learn how to host web apps using servers such as Apache/Nginx etc.

5. After these 4 steps, it then comes down to picking a all-in-one mature web framework (Django for Python, Laravel for PHP etc) to build a boring monolith app OR go cooler with newer technologies, micro services, SPA, shiny new frontend framework (React, Vue, Svelte etc) and more.

I doubt any single source can teach you everything so the trick is to find resources on each topic as you go along. Most importantly, create something and learn every day.

Yes, I know of many. One example is Wes Bos' Full Stack Advanced React and GraphQL [0], where you build your own ecommerce store complete with payments as well.

[0] https://advancedreact.com/

Stripe Atlas has guides on some business side of things. YC Startup School too. Also, makebook.io. And SaaS Metrics 2.0 article by David Skok, google it.

Do you need to build the software to launch?

https://startupschool.org is the first video series I recommend. The videos should be on YT as well

i listen to a lot of indie hackers podcast - i bet if you post a question over on their forums, someone will give you something to look at.

No no way