Is Redis AOF file a mix of AOF and RDB?

I’m experimenting with Redis and digging into the persistence mechanisms.

If I turn on appendonly (appendonly yes) and turn off RDB (save "") in redis.conf file.

Then I start a new database

And set two keys:

SET firstKey "I'm first one"
SET secondKey "I'm second one"

I end up with an appendonly file without any headers, only commands:

raphaeldelio@Raphaels-MacBook-Pro data % cat appendonly.aof 
*2
$6
SELECT
$1
0
*3
$3
SET
$8
firstKey
$14
I'm number one
*3
$3
SET
$9
secondKey
$14
I'm number two

Then, if overwrite the first key with:

SET firstKey "I'm still number one"

And then run BGREWRITEAOF, the file is rewritten and a header similar to the header of the dump.rdb file is added to it:

raphaeldelio@Raphaels-MacBook-Pro data % cat appendonly.aof
REDIS0009?  redis-ver6.2.7?
redis-bits?@?ctimežk?bused-mem˜??
                                 aof-preamble???????֭h
                                                     ????mʗ????~??ױ??firstKeyI'm still number one   secondKeyI'm number two??????֭h
                             ????mʗ??????!4d?%    

However, I was expecting the file to be rewritten as command logs again.

Therefore I have two questions:

1. After rewriting the AOF file, does it become a mix of AOF and RDB file?
2. How can I decode the content of the header?