The following are defaults in Ubuntu 20.04 LTS.

$ ll cron.*
cron.d:
total 40
drwxr-xr-x   2 root root  4096 Jun 27 11:14 ./
drwxr-xr-x 148 root root 12288 Aug  9 22:35 ../
-rw-r--r--   1 root root   285 Jul 17  2019 anacron
-rw-r--r--   1 root root   775 Oct 26  2020 certbot
-rw-r--r--   1 root root   201 Feb 14  2020 e2scrub_all
-rw-r--r--   1 root root   712 Mar 27  2020 php
-rw-r--r--   1 root root   102 Feb 14  2020 .placeholder
-rw-r--r--   1 root root   191 Feb  2  2021 popularity-contest

cron.daily:
total 68
drwxr-xr-x   2 root root  4096 Aug 10 02:47 ./
drwxr-xr-x 148 root root 12288 Aug  9 22:35 ../
-rwxr-xr-x   1 root root   311 Jul 17  2019 0anacron*
-rwxr-xr-x   1 root root   376 Dec  5  2019 apport*
-rwxr-xr-x   1 root root  1478 Apr  9  2020 apt-compat*
-rwxr-xr-x   1 root root   314 Feb 19  2020 aptitude*
-rwxr-xr-x   1 root root   355 Dec 29  2017 bsdmainutils*
-rwxr-xr-x   1 root root   384 Nov 19  2019 cracklib-runtime*
-rwxr-xr-x   1 root root  1187 Sep  6  2019 dpkg*
-rwxr-xr-x   1 root root   377 Jan 21  2019 logrotate*
-rwxr-xr-x   1 root root  1123 Feb 26  2020 man-db*
-rw-r--r--   1 root root   102 Feb 14  2020 .placeholder
-rwxr-xr-x   1 root root  4574 Jul 18  2019 popularity-contest*
-rwxr-xr-x   1 root root   214 Dec  7  2020 update-notifier-common*

cron.hourly:
total 20
drwxr-xr-x   2 root root  4096 Feb  2  2021 ./
drwxr-xr-x 148 root root 12288 Aug  9 22:35 ../
-rw-r--r--   1 root root   102 Feb 14  2020 .placeholder

cron.monthly:
total 24
drwxr-xr-x   2 root root  4096 Jun 27 11:14 ./
drwxr-xr-x 148 root root 12288 Aug  9 22:35 ../
-rwxr-xr-x   1 root root   313 Jul 17  2019 0anacron*
-rw-r--r--   1 root root   102 Feb 14  2020 .placeholder

cron.weekly:
total 32
drwxr-xr-x   2 root root  4096 Jun 27 11:14 ./
drwxr-xr-x 148 root root 12288 Aug  9 22:35 ../
-rwxr-xr-x   1 root root   312 Jul 17  2019 0anacron*
-rwxr-xr-x   1 root root   813 Feb 26  2020 man-db*
-rw-r--r--   1 root root   102 Feb 14  2020 .placeholder
-rwxr-xr-x   1 root root   211 Dec  7  2020 update-notifier-common*

See also cron rules with respect to changing system time, explained in this SO answer, and commented in the cron source here.

Some backup scripts, run by administrator:

# m h  dom mon dow   command
23 4 * * * /usr/backup/backup.sh >/usr/backup/backup.log
23 5 * * 6 /usr/backup/backup-seafile.sh >/usr/backup/backup.log