I have been in this problem for long time and I want to know how it's done in real / big companies project?

Suppose I have the project to build a website. Now I divide the project into sub tasks and do it.

But you know that suppose I have task1 in hand like export the page to pdf. Now I spend 3 days to do that , came across various problems, many Stack Overflow questions and in the end I solve it.

Now 4 months after someone told me that there is some error in the code.

Now by that I completely forgot about (60%) of how I did it and why I do this way. I document the code but I can't write the whole story of that in the code.

Then I have to spend much time on code to find what was the problem so that I added this line etc.

I want to know that is there any way that i can log steps in completing the project. So that I can see how I end up with code, what errors I got, what questions I asked on SO and etc.

How people do it in real time? Which software to use?

I know in our project management software called JIRA we have tasks but that does not cover what steps I took to solve that tasks.

What is the best way so that when I look back at my 2 year old project, I know how I solve particular task?