Extents, Pages, and Row overflow data pages are described in the product documentation:
Pages and Extents Architecture Guide
Large Row Support
Rows cannot span pages, however portions of the row may be moved off
the row's page so that the row can actually be very large. The maximum
amount of data and overhead that is contained in a single row on a
page is 8,060 bytes (8-KB). However, this does not include the data
stored in the Text/Image page type.
This restriction is relaxed for tables that contain varchar, nvarchar,
varbinary, or sql_variant columns. When the total row size of all
fixed and variable columns in a table exceeds the 8,060-byte
limitation, SQL Server dynamically moves one or more variable length
columns to pages in the ROW_OVERFLOW_DATA allocation unit, starting
with the column with the largest width.