A: Hey Sebastien, we're btw. always happy to answer such detailled questions directly in the user chat. :) For now: Overall, as said in other places, too, as the tool is all about handling the complexity of several calendars and provide overview of lots of elements and drag and drop, it is created desktop centered and will be most useful there. :)
Ref. posting to YouTube: Just upload the video file, make sure that your YouTube channel is connected and with the normal respective size it will publish there. It will probably be helpful to check out the YT tutorial about it - and if anything else should come up, we're always happy to help! :)
Ref. your second point: Maybe it will be easier to check in about that directly in the VIP call we're offering. Let me try for now: You can indeed upload lots of post in one - either via the bulk upload button in the post creation window or simply by adding several pics without choosing the carousel option - it will turn each pic into a draft then. For 'centralized view' you have actually several options: Either the 'brand overview' view - that will display all you have mapped on a certain or several calendars and helps keep overview on how the brand message unfolds. The thumbnails in the calendar view also give you this and I suggest to check in and use tags a lot if you have more assets (do as you wish, like in Evernote) - you can then search, filter. etc. for any campaign tag or topic.
For your last question: As the different channels often need a bit different formats/captions lots of people use it that way that they set it up brand A/channel A for example and would see it immediately. It does not filter by channel per se. The way you set up your calendars and maybe post variations can vary a lot according to your use case (for example some people might have a Linkedin and an Instagram version and have those mapped to different calendars). I hope this already helps and as said: Happy to check in more in a quick call to make sure you find all you need and can focus where it's useful.
Happy new year and sunny greets from Munich, Germany!
Kristin [founder + chied product officer Ägile Ässets]