The seller, whether a solo entrepreneur or large enterprise, is a vendor, merchant, product creator, or retailer with a product to market. The product can be a physical object, like household goods, or a service, like makeup tutorials. Also known as the brand, the seller does not need to be actively involved in the marketing, but they may also be the advertiser and profit from the revenue sharing associated with affiliate marketing.
However, people can still do webinars without all of that. For example, you might have a sizable social media following and you train them every week on something to do with social media. But you will need a product to embed and sell at some point. Don't worry about it in the beginning. And by far, the best webinar platform out there is certainly GoToWebinar. Hands down, there isn't even another platform that comes close.
If you have a good idea for an app, and the skills to create it, then producing your own premium app can be profitable. However, with so many free apps available, it may be hard to convince people to pay. If you want to go down the premium app route, consider having a basic free version to encourage people to download the app in the first place. You can then provide the option of upgrading to a premium plan once users realize how useful the app is.