What should I build next after learning frontend fundamentals and Python?

At this point, stop collecting tutorials and start building. Make something, run into problems, look up what you need, and keep improving it. That process will show you which concepts you actually understand and which ones need work. Programming sticks much better when you use it instead of only studying it.

You could combine your interests by building a small Flask application that uses NLP. For example, create a sentiment-analysis dashboard where someone pastes a review, article, or other text and receives its overall tone, common terms, and word-frequency information. A beginner-friendly option is VADER from the NLTK library. This gives you practice with frontend development, Python, backend routes, and NLP in one manageable project.

Pick a small project and use it to test your skills. You’ll quickly discover gaps in your knowledge and learn how to apply the fundamentals in a real situation. Since you’ve already made static sites, a simple backend project with Flask would be a reasonable next step. You don’t need to master every framework before starting.