Like YouTube, Roblox started as an online stage for young creators. As the company grows toward 100 million active daily users, contributors are making big money selling games.

As gaming hits draw tens of millions of simultaneous players, Roblox engineers make sure the internet’s youth never deal with buffering.

Roblox Corp. is testing a short-form video app similar to TikTok that will allow users to capture and share 30-second clips ...