Soft captions are served to the viewer on top of the video and can be generated by tools within some social media platforms. Soft captions are not actually part of the video itself.
Alternatively, within some platforms, captions can be created by uploading a caption text file along with the video file. Commonly used caption text file formats include .srt, .ass, .sbv and .sub files.
Facebook gives you the option of uploading an .srt file and YouTube gives you the option of uploading a number of caption text file forms.
When it comes to LinkedIn, uploading a .srt is the only option available. At present, LinkedIn doesn’t have its own caption generating tools.
Twitter and Instagram currently don’t support any form of soft captioning for video, so hard captions are the only option.