Hi Subha Chanda, I started out with sharing a lot of what I was doing (Learning In Public). I was not focused on just gaining followers, for me with engagement that you'll get that so being genuine is one hack you can employ in building an online presence.
Immediately! As long as you have potential and meet even half of the Job Description. I don't think you should stall because you think you're not qualified. Apply for the job and let them tell you that and you take that as constructive feedback and improve on yourself.
Open Source isn't required for a job in tech. I agree is a nice to have and will look good on your resume but don't let anyone tell you otherwise you can decide to contribute or not and you'll still be eligible for a job.