How does DYNAMIC Row format handle NULL storage MYSQL 5.7? As a newbie to Mysql I just can't understand how NULL values are stored in each row formats.

As per docs(last bullet-in), NULLs for fixed length columns (BIGINTs) also take their data space reserved. Meanwhile, NULLs for variable length columns are handled differently. The behaviour is defined under REDUNDANT row format description. I doubt what will be the behaviour with DYNAMIC / COMPRESSED row formats?

Note: I have tried to create 997 bigints in a table and have inserted value for one column. I had around 1957 rows but only had 26 pages, which means NULLs doesn't occupy space for fixed length. Am I understanding it in correct way? Please anyone help! 
Specifications: 5.7.23 | Innodb | Dynamic Row | 16kb page size
The second bullet point 'The variable-length part of the record header contains a bit vector for indicating NULL columns'.

So when a row has a field that is NULL, the bit in the bit vector of the header is set.

AFAIK this is the same for DYNAMIC/COMPRESSED that really only changed the handling of variable length fields.