I think it really depends on what you are trying to achieve. Frontend itself is pretty diverse and you could easily get overwhelmed learning everything.
At the core of it, web development/Frontend is about:
But you would need to learn about the various framework modern web pages are build on, as well as how the various SaaS ecosystem work with one another, SSO with Okta or Google for example.
My journey learning Frontend have been pretty unconventional and scattered, but there are Udemy course like "The Web Developer Bootcamp" that I heard a lot of good things about, but I have not tried that myself.
I did use "The Road to React" by Robin Wieruch to ramp up in react and used it in my professional work.
I agree it really depends on what you want to do.
If you don't know HTML, CSS, JS I would start there.
A bit off the beaten path: I would check out https://elm-lang.org. I'm not suggesting implementing your SAAS in Elm, but learning Elm if you want to learn a mental model for React. IMO, Elm uses the same concepts (declarative updates, explicit side effects) without the boilerplate of React.