Great question! You actually squeezed 3 topics in here, which are all inter-related and important to buyer journey maps. In my experience, I start with segmenting my audience (total addressable market by country, company, vertical/industry, etc), identifying personas within my target audience (buyer/user), mapping the journeys for each persona, and then testing which content formats/channels work best. This means if I'm selling to Enterprises vs. mid-sized companies, IT vs. Finance buyers, I should understand the different journeys, budgets, and decision making processes and plan my content accordingly. Be choosy about where to focus!
I would test and measure content starting from the top of the funnel for awareness (paid ads, SEO, partnerships, brand campaigns, podcast, etc.) - is your brand a consideration for your category? Move to the middle to drive consideration (web, case studies, white papers, demos, etc.) - is there preference for your brand vs. competitors or alternative offerings? And finally, bottom of the funnel to close the purchase (talk to sales, referrals, trial, etc.) - are you the right choice to meet their needs? Content at each of these stages has a different intent and measurement for success. The more specifically your content addresses a specific buyer's problem with the right level of information, the more likely you are to convert them to the next stage!
From a research perspective, I've found a few research frameworks to be helpful for journey maps: jobs to be done, diary/desk studies, and 1:1 interviews. That's why getting the right person from the beginning is important, and meet with as many as you can until trends become apparent.
Best of luck!