There are tons of online resources to learn Dart and Flutter for cross-platform mobile apps, desktop, and web app development. Some of the notable resources which I could list out are.
Dart Programming Language
Flutter App development
What are you waiting for? Go and get your hands dirty with Dart and Flutter and become a pro-Flutter dev with side projects and build your portfolio.
Keep sharing your learnings and your projects built with Flutter.
Happy Learning under lockdown!!