Sponsored posts work much in the same way as paid guest posts, but they are posted by big businesses instead of individual bloggers. Therefore, the scope for fees is much higher, as businesses have larger marketing budgets than humble bloggers. Having sponsored posts by large companies will also help promote your site as reputable and as a leader in its field.
If you’d like to specialize in medical transcription, there are certification courses you can take; you can also benefit from having healthcare-related experience so you’re familiar with the terminology. The same applies for legal transcription – you can do a certification course, and it’s helpful if you know legal lingo or even have legal reference materials at home. If you’d like to do law enforcement transcription, that most often has overlapping requirements with legal transcription.
The topic you choose must have enough depth that you can create a lot of content for it. This is important for building an authoritative site, for search engine optimization, and most importantly, for the end user. If you don't have enough content about a topic, you're not going to be taken very seriously as an authority on the topic and it's unlikely you can convince someone to make a purchase from you.