Help to symlink two Drives

What exactly do you want to achieve?
Which files/directories reside where?

I read Symlink points to a directory or file and this what I want achieve.
On Ubuntu installed on SSD I have the normal directories Documents, Downloads, Music,Pictures.
On HDD I have the some directories with my data I want from the SSD to open the HDD directories to.
Example: from the bootable SSD Documents directory I want to get the HDD Documents directory to work with files in it.
I tried but things dint work properly.
Need an example.
Thank you

Asking a question like "how do I apply the symlink command" is premature and goes already a step too far.
Before we can help you, you have to tell what your aim is. Maybe there are much better ways than using the symlink command.

What is the structure of (the main partition of) the SDD drive? Is it a standard Ubuntu structure with /, /home/, /home/username/, /home/username/Downloads/ etc., or do you have some part of the directory tree in separate partitions?
Are there files in the Documents, Downloads, Music... directories on the SDD?
What is the structure of the HDD drive? How and where do you mount the HDD partition(s) into the directory tree?

Why do you want to use the slower HDD drive, if you might be able to use the faster SDD for the files and directories that you mention in your question?

The HDD is my daily working drive with standard installed Ubuntu it has all my data, pictures,
movies etc.
The SSD drive s it a standard Ubuntu structure with /home/username/ newly installed
Ubuntu there no data in Documents, Downloads, Music... directories.
I want to use the SSD to boot into Ubuntu and use the data from HDD directories.
Example: boot SSD open LibrOffice Calc and then get directories contained my Calc files
to work on them.
I hope this explains

You did not answer my questions.

The idea was to don’t over use SSD with files that they are edit and changed daily.
I view online that Symlink makes a reference of the Folder/Directory I tried
few times and outcome was a errors or found it copies all the from one Directory
to other and this is not I thought to achieve.
Please advice if any other method.

What is the structure of the HDD drive? Is this also a standard Linux filesystem structure with /, /home/, /home/username/, /home/username/Downloads/ etc. (e.g. stemming from a previous Ubuntu installation on the HDD)?

How and where do you mount the HDD partition(s) into the directory tree?

Remark: What you want to achieve (system files on one partition, user data files on a different one, in this case even on a different disk) often is done by creating a separate partition for /home and automatically mounting it at boot time.

Structure of both drives are ext4 with /home/username/.
Don't understand the second question.
System files are in SSD and /home is the HDD witch is automatically mounting it at boot time.

What is the output of the command
mount | grep '^/dev/'

luis@luis-System-Product-Name:~$ mount | grep '^/dev/'
/dev/sda2 on / type ext4 (rw,relatime,errors=remount-ro)
/dev/sda2 on /var/snap/firefox/common/host-hunspell type ext4 (ro,noexec,noatime,errors=remount-ro)
/dev/sda1 on /boot/efi type vfat (rw,relatime,fmask=0077,dmask=0077,codepage=437,iocharset=iso8859-1,shortname=mixed,errors=remount-ro)
/dev/sdb2 on /media/luis/6f014f0b-7ccf-410c-9bcf-bf911baac89f1 type ext4 (rw,nosuid,nodev,relatime,errors=remount-ro,uhelper=udisks2)
luis@luis-System-Product-Name:~$

sorry, I missed to ask earlier: what is the output of the command

cat /etc/fstab

luis@luis-System-Product-Name:~$ cat /etc/fstab
# /etc/fstab: static file system information.
#
# Use 'blkid' to print the universally unique identifier for a
# device; this may be used with UUID= as a more robust way to name devices
# that works even if disks are added and removed. See fstab(5).
#
# <file system> <mount point> <type> <options> <dump> <pass>
# / was on /dev/sda2 during installation
UUID=95a06925-d082-49f9-9e18-4453d708acbd / ext4 errors=remount-ro 0 1
# /boot/efi was on /dev/sda1 during installation
UUID=557A-36B4 /boot/efi vfat umask=0077 0 1
/swapfile none swap sw 0 0
luis@luis-System-Product-Name:~$

Thank you for great help and explanation.
I want to get this working but don’t enough knowledge I need help, must I open a new
question or can I carry one with this one?

Thanks Manfred Hampl, that solved my question.