What do the vertical lines in the dock signify?
I have two lines on my Dock that divides my Apps into three sections.
What is each section for?
Dividers.
Right side is folders you can add, I suspect middle is launched apps (recently used)or some such, and left is Applications?
If there was a far left side— hand off or continuity.
Use the Dock on Mac - Apple Support
Change Dock preferences on Mac - Apple Support
Pick up where you left off with Handoff on Mac - Apple Support
Use Continuity to work across Apple devices - Apple Support
Command Tab keys opens the heads up display (HUD) of all running applications.