Hello Piyush! Once you have worked for 3-4 years, you have some relevant experience in the field and after knowing the basic knowledge to acquire more consolidated knowledge you have to take up more projects and manage more projects and all the challenges that you will encounter will make you learn and derive more knowledge. To gain professional knowledge in any field you have to work and you get experience, after constant projects. For more such insights, stay tuned to Seekho; we keep coming up with courses to enhance your knowledge.

I am not from this field but check out this article https://www.productplan.com/learn/resources-for-product-managers/ I thought this was helpful!