When creating closed captions for YouTube, you need to upload two files: a video file and a subtitle file. Captions lets you auto-generate subtitles, edit them if needed, apply the appropriate font, and download the final SRT file.
Steps to follow:
Tap Export in the top right corner of the screen while viewing your project.
Choose the optimal resolution, frame rate, and tap Export video.
Do not close the window while a project is exporting.
Tap SRT on the bottom panel, and choose the desired destination.
All done!