Please see this post about mood boarding. Essentially, Mood Boarding is the foundation of many designer's process (RadNolan's included).
It is a great tool for client/stakeholder alignment. See, a client has a vision and it's your job to bring that vision to life. Unless they are very articulate or trained in visual design, they will likely not be able to give you great insights as to what exactly their vision is. This means you have to show them options and hope for the best. But through mood boarding, you can show other people's options without spending a huge chunk of the budget going in the wrong direction and leaving the client/stakeholder stressed out that you won't be able to do the job.
Beyond that, Mood Boarding it a great designer's-block tool. Sometimes you just can't seem to get something right--you have an idea in your head but it just isn't working. That is a great time to stop and look up more inspiration to get a clearer idea of that idea.
Focus on Layout, not Aesthetic
Once you have the User Flows mapped out, you will be able to have a rough idea of the types of interactions you'll need. With that, you can go online and find great inspiration.
You'll think that a card slider may be good for this information while a list view is better for a different set of information. That's what you're focused on here: taking layouts that you have in your head and seeing them visualized.
If you're working with a client, you'll often have to throw in a few moods that are purely for aesthetic, but aim for ~80% of moods focused on interaction and layout.
Group by Screen or Section
When you have layouts an interactions mapped out, and you find the right inspiration for it, you'll screenshot or copy the images and drop them into Figma grouped by the screen or section they're addressing. But avoid getting distracted by cropping the ui shot to just the part that you think works for the section.
Then you'll want to add notes near the sections so you can remind yourself or tell the stakeholder/client exactly why you're using that design and what you see that translates.
Lots of people hate on Dribbble, but RadNolan loves it for what it is: use cases of UI. See, you're not going to Dribbble to decide on User Experience patterns. You already mapped that out in the User Flow Mapping stage. Here, you're looking for inspiration to get a clearer picture of the layouts and interactions for your designs. That's why Dribbble is where I begin in Mood Boarding
This is great for finding exact types of moods. When you're Mood Boarding, don't think about the industry you're working on. Think about industries that might have the layout you're going to use in your designs. RadNolan once closed a project with a Mood Board for a tutoring app that included designs for an investment trading platform. RadNolan found that by searching on Pinterest for investment dashboards and looked for the layout that he had in mind.
Behance
This is a great tool where designers often share full designs of their apps. This means that you can often find better UX patterns here because they've been stress-tested during the process of designing the full app.
Mobin
This one is great for finding REAL designs. It's basically screenshots of live products. RadNolan uses Mobin primarily when he's got designer's-block.
