Level 1
Hi there, it's perfectly fine for you to post here for UGC questions, but anyway the reason why you may see missions with 100+ plays, is because when you upload/re-upload a mission it is considered as 'Newest Missions'.
For example, a player with a created mission that consist of 136 plays wishes to make a few changes in that particular mission, let's say the player adds more enemies to make it harder, however these changes won't take into effect only if you re-upload that mission, so the player re-uploads the mission again with more enemies and the change is now permanent and everyone who plays will experience it the way the mission was edited. As a result of re-uploading the mission, it is placed in the Newest Missions filter, however the player's Plays/Favorites/Reviews is still retained, therefore there's still 136 plays. So in order to make permanent changes to your mission, you must once again upload your mission, but it will be put in the Newest missions filter and you will still have your plays recorded. To test this try re-uploading a mission, then have this site opened and click 'UGC missions' under 'Community, then click 'Recently Published', you will most likely see the mission you just uploaded to the server.
Yes, missions that are uploaded can be played right away, it may seem at times that it's not uploaded yet. The reason why you may see different missions every time you log out, is because there are thousands of UGC missions that await to be played, this is important see, because when new UGC missions pop up from the filter it gives players the opportunity to try those missions, it cycles to different missions that may be played or not played every time you log out, even if you change the Mission filter it cycles as well. However if you are looking for specific missions that is what the 'Search' option is for. If you wish to play a mission that catches your eye, but you want to play later, you can 'Queue' the mission and save it for later, this can be done in game and on this site here under UGC missions as I've mentioned.
I hope this helps you have a better understanding of how UGC missions work. I also recommend following what RetroSteveUK mentioned to have others play your missions. 
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