Method was kindly shown to me by user Mefody
in answer to a thread
Rooting SM-T380/T385 via Magisk Manager APK v.5.5.3 & Magisk 15.2:
1. Unpack stock firmvare ***.tar.md5 that is currently installed on your tablet (they MUST BE the same!!!).
2. Then unpack AP-block from step 1 to obtain "boot.img" file.
3. Download & install on tablet Magisk Manager APK.
4. Put "boot.img" on SDCARD.
5. Run Magisk Manager, in setiings set "Patch kernel", then setup the Magisk zip (Wi-Fi must be connected to Internet ).
6. When asked, point Magisk Manager to place on SDCARD where "boot.img" saved.
7. After some time You will get "OK".
8. Find on SDCARD file "patched_boot.img". Copy in on PC & rename to "boot.img". Then archive it to "<any name>.tar".
9. Set tablet to download mode & flash above tar-file via AP-slot of Odin-programm.
10. Tablet will reboot & ask to wipe all data to factory's params. Do it... Be noticed, that all passwords, setting, google registry will be wiped.
11. After wiping & rebooting install Magisk Manager APK again... And YEAH!!! You got root!