Do you have any snippest in C# how to export footage with player from Milestone Server based on REST API?
No, unfortunately there is no functionality in the Rest API to export footage (not yet, there are future plans to expand the Rest API).
My best advise is to look at the Mobile Server and Mobile SDK for generating and downloading exports.
Does Miblie SDK is enclosed in all free versions since 2021R3?
yes
Where can I find openapi.yaml out for Milestone server 2021 Moblie SDK?
There isn’t one. The Mobile SDK is not that kind of API. Unfortunately. I hope I haven’t mislead you.. https://doc.developer.milestonesys.com/mipsdkmobile/index.html?base=content_1.html&tree=tree_1.html