If you just want experience - as the others have already said - contribute to OSS. Otherwise, take paid internships or trainee jobs. Don't give away your time for free in hopes of moving up - you'll end up undervaluing your own work later.
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We're a place where coders share, stay up-to-date and grow their careers.
Don't.
If you just want experience - as the others have already said - contribute to OSS. Otherwise, take paid internships or trainee jobs. Don't give away your time for free in hopes of moving up - you'll end up undervaluing your own work later.