How do you suggest transitioning into a different PM field? (E.g. if you're an internal PM, and you want to be a B2C PM, or growth PM, or B2B PM, how do you go about gaining the needed skills/making yourself more marketable?)

Congratulations -- you are already a PM, so you're starting from a position of strength. Regardless of whether you are an internal PM, platform PM, B2B PM, B2C PM, etc. there are always common PM skills across these areas, so think about what is transferable and what is not, and emphasize those. For example, if I am hiring for a customer-facing PM role and I have an internal PM candidate in front of me who is trying to make the case that they are the best candidate for that, I have to be convinced that they can do customer discovery with paying customers who aren't going to be as nice/straightforward/cogent/etc. as their previous customers (i.e. their internal team co-workers who really can't fire them). Emphasizing how your experiences from your originating domain will apply to the target domain is key.

Beyond that, there are plenty of tactics and resources like courses on Maven, Reforge, etc. to teach you how to do a different PM field.

One last piece of advice: set your promotion/career growth expectations appropriately. For example, if you are currently a growth PM and on-track to get promoted in a year, don't expect that if you make the transition to an external B2B PM that you're going to necessarily be promoted on that same timeline. You need to spend some time to get up to speed on the area, develop your skills, leverage what you know and learn what you don't, etc.