This is a tricky question. If you post using any platform that builds on hive, it posts to the same blockchain layer. However... different platforms may have their own logic on what you can see.

Peakd and hive.blog let you see everything. So if you post using vimm.tv, or 3speak, or leofinance, you will see it on peakd and hive.blog, but they can only see each other's content if you use the right tags, and maybe not even then. (Leofinance will let you see vimm.tv posts with finance tags, but vimm.tv might only let you see content that was actually posted/streamed using vimm.tv.)
And then of course there is actifit, and pob, and dozens of other sites that all use the same blockchain but have their own layering and rules on what will show and what won't.

I mostly use peakd (I never use hive.blog) but sometimes I will use a community platform for specific post types (vimm.tv for streaming, for example), but then continue to manage all my comments and responses on peakd.