How to accumulate data in one cell?

Question

Google Sheets

According to the formula =mid(importxml(x; y);a;b), values ​​from the site are displayed in the cell.
It is required to add (not replace) data to the cell by automatically running the formula periodically. Data must be provided in the form of a drop-down list, sublist, free table. The time and date of the check should be recorded in adjacent cells.
How to implement?
Google Sheets app or Google Sheets

Answer 1

1) read into buffer
2) append to buffer
3) write buffer to cell