All marketplaces are platforms. Hence, Clarity is a marketplace and a platform.

A platform can be defined as a place of interaction where users with freedom of decision making power interact with each other, and the platform owner defines the rules of conduct and governance.

On the flip side, not all platforms need to have marketplaces operating on it. As an example, Microsoft developed its Windows operating system for 2 decades as a platform without a marketplace. It was a platform because app developers created many software applications for Windows OS users to install and use. It wasn't a marketplace because Microsoft didn't operate any kind of a medium for app developers and users to meet and transact with each other.

Compared to Windows OS, Apple's iOS is a platform with a marketplace built on top: app developers and OS users meet on App Store to interact and transact.

I would recommend tackling a reskilling journey like this across 3 dimensions: (1) principles (2) learning / skill acquisition (3) applied learning

Principles are what govern your mind and behavior about a challenge. These are not specific to the UI/UX domain, but valid for all similar cases where one wants to reskill / upskill.
1) Growth mindset
2) Grit, persistence
3) Turning learning into habits

On the 2nd dimension, learning and skill acquisition:
I would suggest engaging with a learning program that enables you to apply what you learn within the course. There are many self-learning platforms out there, such as LinkedIn Learning, Masterclass, Udacity. I would go for one where there is an opportunity for either your peers or the course instructors to review your submissions and give feedback. The second part of this dimension is choosing specific skills to focus on. Adobe's suit of design software is usually the safest bet to start with. The third part is the community aspect. In our reskilling journeys, we benefit a lot by triangulating with others. Joining communities such as dribbble will help you understand how other designers are iterating on their designs and how they evaluate each other's work.

The last and third dimension is finding ample opportunities to apply what you learn. Many perfection-minded people focus on pure learning and absorbing until they try their hand at doing something many months down the line. The fact of the matter is that design is very much an applied skill, and therefore you gotta find opportunities to test your newly acquired skills in the real world, preferably early and often. I would consider submitting your works in design communities to get feedback. Getting things right or wrong is a lot less important than being able to get feedback so that you can triangulate and fail forward. At a slightly later stage in the journey of applying what you learn, I would become a service provider on fiverr, upwork, or linkedin to take on projects and work with clients.

Best of luck with your journey!

When building a platform out a platform, the most important aspect initially is to be able to create core interactions among the initial users. These core interactions make the platform valuable, and vastly increases your chances of developing viral growth loops, as opposed to word-of-mouth effects.

To help you distinguish the difference between viral growth and word of mouth, here is a thinking.
Word of mouth will happen when somebody hears that Mark Cuban is on Clarity and she will tell her friends to check Clarity out; it is newsworthy. You will get sign ups out of curiosity, but those new sign ups will still need to interact within the platform to create value.
Viral growth will happen when users get more benefits by having their own social connections on the platform, and therefore they will invite their friends to join. For example, if Mark Cuban was on Clarity and Clarity was offering a "group chat", a single Clarity user might decide to invite her entire startup to join Clarity, so that each can connect to the group chat with their own accounts and ask questions to Mark. This positive network effects differentiate viral growth from other traditional marketing effects, and viral growth relies heavily on the existence of core interactions in the platform.

Finally, here are tiers of value your producers would create early in a marketplace
Most valuable scenario: You have high-profile producers and they interact. I.e. Mark Cuban is on Clarity and he takes calls and answers questions.
More valuable scenario: You have producers (high-profile or low-profile, doesn't matter as long as they can produce value) and they interact.
Less valuable scenario: You have high-profile producers but they don't interact. In this case, more producers would continue to sign up to become associated with Mark Cuban, hence you get some viral growth. You would rely on your low-profile producers interacting.
Least valuable scenario: You have low-profile producers but they don't interact. This would signal the future demise of the platform unless you step-change the core interaction volume in the marketplace.