How to write on sdcard

I have an sd card on my Xperia X with SFOS and I want to create directories and copy files on it.
But I have a “permission denied”.
I see “/media/sdcard” directory is a link to “/run/media/nemo” and this ones is writable only by root.
So the problem become how can I run commands as root?

I made manually a link to /media/sdcard in my /home/nemo directory. This works and shows there a folder 0123-4567 what can be opened as user nemo and is the SD-card. So writing to the SD card is possible.

Is your username nemo or defaultuser (/home/USER)?
I’m nemo, and nemo is the owner of /run/media/nemo and has write permission (drwxr-xr-x). So I don’t have any problems in writing to sd.
To become root enable Developer tools in Settings. Then Terminal appears in the app list, and in the command line give devel-su ls to run ls as root. Or devel-su without args makes you root for the terminal session.

Both the suggestions are very good.
Thank you.

The user directory in the file system is still named ‘nemo’ despite the new ‘defaultuser’.

Everything works better if ‘defaultuser’ or ‘Eigentümer des Gerätes’ is renamed to ‘Nemo’ as it was before. Less buggier then. Don’t know why, but screen freezes less frequent now.