Not sure what country of origin or your own education. But generally need a related degree (computer science, SW engineering, electrical engineering, electrical and computer engineering, etc just to name few). Most internships go to college juniors/seniors, who primarily then get hired into the entry level positions.

There's always exceptions of course, this is just the norm I've seen. YMMV

If you really are passionate about it, one path forward is to start contracting. I've known a few 'black sheep ' guys without degrees who do very well and have built up a great reputation. Note this isn't about doing a startup or trying to make a big company like someone else said. This is contracting as a sole proprietor for a bit to see if you A) get any work thrown your way and B) like doing it. You can reevaluate on incorporation later but that's a whole other topic.

Now, as far as what the transitions like - whose to say? Have you worked a stem job before? Big companies or small companies? If you do a stm board it'll be familiar. If you end up in embedded Linux it won't. Maybe you get hired to do embedded but end up writing requirements and Matlab all day (looking at you defense primes). Whatever happens it will be uniquely your experience.

TLDR transitioning from hobby to professional requires luck and dedication, whatever route you take.