Visa Requirements for Short Courses

What do you mean by bootcamp? And a short term Schengen visa doesn't allow work, only for tourism, nor can it be extended beyond 90 days in a 180 day period.

You need a job seeker visa or to line up a job and get a work visa. A freelance visa is theoretically possible but not easy. They will want to see a business plan and enough start capital and understand how you can make it as a freelancer. This usually means knowing the local language, having not only experience but also connections for networking in your branch. Otherwise, what chance do you have?
Do you speak German. Some opportunities exist for English speakers but only in niche fields. Normally English is good - but in addition to German rather than as an alternative.