How to transfer all data from Mac to external SSD

First of all, before you can do anything with the disk, it is advised that you erase and reformat it for use with the Mac.

Erase and reformat a storage device in Disk Utility on Mac - Apple Support

If it is your intent to use this as a backup disk, then set it up to be use by Time Machine.

Back up your Mac with Time Machine - Apple Support

If you simply want to move a copy of your data to the drive, you can drag and drop your user folders to a folder that you create on the new drive.

Connect and use other storage devices with Mac - Apple Support

If you intend to use the new drive as a startup disk for your Mac, then you'l have to install macOS on the drive and then use the Setup Assistant or Migration Assistant to transfer your stuff. This option is pretty involved, so let us know your intent and someone here will provide additional guidance.

Note that if you move your Photos, Music, and/or TV libraries to an external drive, you may need to go into those applications and tell them to use the library on the external drive.

With all three of these applications, if you hold down the Option key while launching the application, a dialog will come up that lets you create or select a library. If you are using iCloud Photos, only one library can sync against iCloud. You can change which library is the System Photo Library by starting up Photos in the new library, going into Photos > Settings… (General tab), and clicking on "Use as System Photo Library". (It will be disabled in the event that the current library is already the System Photo Library.)

Move your Photos library to save space on your Mac – Apple Support

Apple Music User Guide – Change where your music files are stored on Mac

Other World Computing – Rocket Yard Blog – Move or Manage The Music, Apple TV, and Podcast Libraries

Why do you want to copy all of your user data to the external?

What Mac model and year do you have?

What system version are you running?

How big is your boot drive and how much free space do you have in it?

The only user data that should be move to the external are the Pictures, Movies, Music and Documents folders and their contents.

After copying the folders and contents to the external drive make sure the items in the copied folder work as intended before deleting the contents of the original folder. DO NOT delete the original folders, only their contents.

Personally I keep my Documents folder on my boot drive and not on my external drive(s).