I would also like to cast a Youtube video from my PC to my Mi Box.

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This should be trivial, the Mi Box S has Chromecast Built-In. You should just be able to cast from Chrome on your PC. Works fine with my Shield.

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It is not just a matter of a keyboard, I would like to copy/paste username/password/urls etc in my Kodi/Netflix and so on, or copy/paste links to my Youtube for example.

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You probably won't be able to accomplish this, unless you want to do something like copy them into a text file in a shared network drive/folder, then access that folder on your Mi Box S and copy them out into wherever you need them. Seems like more trouble than it's worth without really accomplishing anything.

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You should be able to either have your laptop serve up a drive/folder as network storage that you could mount on your Mi Box, or alternatively have your Mi Box serve up some network storage that you could access on your laptop. Either way should work for file transfers to/from each device.

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It is not just a matter of a keyboard, I would like to copy/paste username/password/urls etc in my Kodi/Netflix and so on, or copy/paste links to my Youtube for example.

This can be done from your phone using the Android TV Remote app, but I'm not sure if there's a way to do it from PC.

I would also like to cast a Youtube video from my PC to my Mi Box.

This is definitely doable. Android TV devices (including MiBox S) have Chromecast Built-In, meaning anything that can cast to Chromecast can cast to your MiBox.

You can use samba share for file transfer, i recommend solid explorer app for android (supports smb v2/3),
You can use android tv remote app and kore app for copy/paste stuff,
You can cast youtube from chrome web browser on pc, just press chromecast icon,
Also kodi 18 has smb v2/3 support too.

I use an App called Vysor or teamviewer but Vysor is the cheapest option. For large file transfers I have my Android boxes setup through my FTP client put a App called MyFTP on box this turns it into a server. You can set user pass and also select what path you want access to and option to make writable it's fairly simple I did make a video on how to do it but am not sure on Reddits policy for posting outside links. The Vysor app allows you to control and record also drop apks/zips it's really a great App I use it quite regularly.

https://play.google.com/store/apps/details?id=com.joaomgcd.join&hl=de&referrer=utm_source%3Dgoogle%26utm_medium%3Dorganic%26utm_term%3Djoin+android&pcampaignid=APPU_1_HJimXOqyFIvRwQKVxbKwCA

Get scrcpy : https://github.com/Genymobile/scrcpy/releases

With it you can have your android tv mirrored to your pc. You drag and drop files on the mirror and it goes directly to you android /download folder.

You drop a ipk and it will install it. You can type from your pc and control it totally.

https://github.com/Genymobile/scrcpy

For simple copy/paste for users/links and so on I found that the fastest way is pushbullet. You can sinc more devices and even send files between them no matter the OS.

with VLC instaled on your miBox you could stream videos or photos if you have a shared folder on your PC.