Hello, Hafunerix,
For future inquiries, we kindly ask that you initiate new topics for each question you may have by visiting our support forum: https://www.8theme.com/forums/xstore-wordpress-support-forum/.
Regarding the question you have posed, Dashboard > XStore > Static Blocks > Create a static block with the desired content, then go to Single Product Builder > Tabs > Use Static Block for the custom tab.
We hope this information is helpful.
Warm regards,
The 8Theme Team