Creating a post in a Space will be different depending on which Hub Layout you have chosen - Video, Discussion or Chat Layout.
Video Layout
To create a post in this horizontal layout, go to the right and select Post.

You will then be asked which type of post you would like to create – Text, Record a video, or Voice.

Discussion Layout
To create a post using this vertical layout, go to the bottom right and select the purple +.

You will then be asked which type of post you would like to create – Text, Record a video, or Voice.
Chat Layout
To post using this layout, you can simply begin typing and click the green send arrow to post. You can post using text, video or voice.

See this article for more information on how to reply to posts using the different layouts.
