Is there a way to play videos over a wifi connection (tethering/direct)? I'm using a Nexus 9 which is lacking a SD card slot. Therefore I want to play videos from my Android phone which has enough space. As I need it for my vacation it should be an offline solution. The solutions I have found only providing copying the files from one device to another (Airdroid,Xender,Droid over wifi). Is there any streaming/direct access solution?
Run an FTP or HTTP server on your Android device which has bigger storage. There are many apps on Play Store which can create a server on Android like FTP Server Ultimate or Servers Ultimate Pro. Creating a server on them is very easy. You can get an idea from here as well (see #Method 3).
Install a file manager which allows access to remote storage such as ES File Explorer or MiXplorer. Both are very intuitive and can create an FTP server or either access it. (See step 5 here for former's usage.)
It goes without saying that both devices need to be connected under same Wi-Fi or hotspot.
Install a multimedia player such as MX Player to stream videos.
A more user-friendly solution might be to use AllCast together with the AllCast Receiver.
AllCast allows you to cast your media from one device to another. Usually, it's used for casting to streaming devices like the Chromecast and Smart TVs, but if you install the AllCast Receiver, you can also cast from one Android device to another.
As with running a HTTP server, both devices do need to be connected to the same WiFi network.