Most of the apps will rotate when I turn my device. So does the screen on my tablet. However, on the phone (Galaxy 9), that doesn't happen, making usage in horizontal mode a bit awkward.
Is it possible to make it rotate and if so how?
I am currently using a Galaxy S10+, and I was using a Galaxy Note 8 before that. Both phones support rotating the homescreen to landscape in their native launcher, but it is disabled by default. I am going to assume your Galaxy S9 supports this as well. Try the following:
Go to Settings > Display > Homescreen. Then look for a setting related to rotation.
On my S10+ the setting is called "Rotate to Landscape Mode" and you have to switch it ON to enable homescreen rotation.
On my Note 8, the setting is called "Portrait Mode Only" and you have to switch it OFF to enable homescreen rotation.
I'm not sure regarding the default UI settings as I use Nova Launcher myself (on a stock Samsung Note 8). For Nova Launcher though -- root not required -- you would go into Nova Settings > Look & Feel > Scrren Oreintation and set to desired settings, such as Auto-Rotate. I noted that the default value is set to "System Default", which does not rotate. In the default device settings, navigating to Display > Home should be more options; for me it just launches the Nova Settings, so unfortunately I can't test it for you at this moment, but I hope that helps!
EDIT: I just tested on my spare device and found the default system launcher UI (Samsung Experience Home) does not offer a method to rotate your home screen by default. However, if you update your OS to 8.1, the option toggle "Portrait Mode Only" should become available as an option (source). Alternatively, if you are willing to dive in further into backend settings, you may be able to use an application like Setedit to modify it, but I need to do some more research to verify that.
Quoted from an earlier answer of mine:
Ultimate Rotation Control forces any app - even those that don't support a certain orientation - into any orientation. It also supports setting on a per-app basis.
However, note that even if you could force an app to show in landscape, the app itself might not actually support it, and the layout could look somewhat messed-up or downright unusable. The aforementioned app is paid but has a free 7-day trial for you to evaluate the situation with.