I'm going to simplify a lot, but here would be my suggested strategy:

• Pick a solid project you want to contribute to
• Start learning the technology (i.e. the programming language) that the project is written in
• Create multiple toy projects in the same technology while learning it (while watching youtube videos, doing online classes, etc)
• Export all of those projects on your github
• During all of this also focus on "Computer Science 101" - learn data structures (especially HashMap) and common algorithms
  • implement all of those in the same language and publish on your github
• When you feel comfortable with the technology, reach out to the companies that you would like to work for (if you want big company, then watch youtube videos on interviewing... the types of questions are quite similar across the board)
• You could try Q&A roles first, which have less programming requirements

You may need to do 10+ interviews, but that's normal.

Overall this could be as fast as 4-6 months if you are starting quite from scratch.