It's relatively simple to do an MVP (minimum viable product):
- Make a login system (or use an existing one like social signon)
- Tie that into a preexisting billing system like paypal etc
- Let people upload videos
- Return a list of videos
- Search by video attributes
- With automated testing
- Revision control (git)
The invisible parts of the app are harder:
- Adding user settings (forgot password, change email, etc)
- Scaling the system across multiple servers
- Uptime monitoring
- Database backups, integrity verification, & restorations
- With adequate database capacity and caching systems
- Across many different languages hosted in different regions
- Copyright protection making it difficult for someone to steal the videos
- Dealing with SEO (search engine optimization)
- Member marketing (emails), CRM (customer relationship management)
- Cost management (servers, etc)
- A/B or multivariate testing
- User requested improvements
A game (pre-alpha) prototype would be relatively simple.
A fully functional game would be harder due to:
- Anti-cheating
- Realtime syncronization of multiplayer issues
- Animations, data, and refresh rates
- User interface design
- Game settings (keyboard control mapping, etc)
- Story, art, music, & other types of content
- Gameplay balance issues
- and many more items
Before you fully commit to building a game, I would highly recommend reading these items just to see how complex seemingly simple problems can be.
I also recommend you start with one small portion of a game system such as a map or minimap or terrain tiling. Even a small system like that can take a generous amount of time to create or implement using plugins.
Games have a wide variety of complexity, anywhere from Pong which can be built by one person in a day to MMOs which take hundreds of people a decade to build. Saying "I want to build a game" is a difficult thing to realistically plan until you have a good idea of what you want to build.
Finally, I recommend you create an outline document of what systems are needed and what you hope to accomplish. That will help you estimate how much time it will take to make a project.