Can’t really say for mlem, but when using memmy: visit !main or !blind (or whichever community you choose), the post button is the last one in the row of buttons under the community name. It goes Subscribe — About — Post. I also checked and it seems to work correctly with voiceover.

I wondered this too but I think it’s fairly simple. It looks like you head over to the community you want to post in and then you’ll see a menu in the top right with the option to post and other choices. It seems a little more intuitive in the mobile browser but that should work

Hello there...I am beta testing Mlem for iOS...I'm not sure that I know the answer to your question. However, you may want to direct your inquiry to the Mlem community here -> https://lemmy.ml/c/mlemapp