According to my perspective,
You can go through the "Adobe Certified Professional - Adobe Commerce Developer" Certification Exam syllabus.
https://experienceleague.adobe.com/en/docs/certification/program/technical-certifications/ac/ac-professional/ac-p-developer
This exam covers beginner to advanced developer-related topics.
This way you can explore Magento.
Additional tips would be:
Learn the file structure of Magento and what you can find where
you need to reference the core. vendor
folder will be your professor.
Learn how a theme and a module are supposed to be structured in the
file system. Then pick one to learn first.
You can download some free Magento extensions, Check how their
developer code, how they implement code, how they fulfill the
requirement.
Learn to debug, If you are good at debugging, You will definitely
pro yourself in development and customization.
Install Xdebug, and Magento Coding Standard Tools, Learn & observe how
things work in Magento in foreground and background.
Once you've picked one, pick a project based on that part of Magento
for example: changing the design of the category page or creating a
module to display a maintenance alert modal with data from the admin
console.
You can also take reference of any e-commerce site, go through the
the site, List out functionalities that the site has, Prepare modules and
try to implement it in Magento.
Again, It's based on my experience, everyone learns in different ways.