Platform overview
Start here. This page explains how the whole thing fits together, so the App specific pages make sense.
What this is
One platform holding several business Apps that share the same customers, the same people, the same files and the same bill.
You turn on the Apps you need. Everything you do not turn on is simply not there, which is why two workspaces can look completely different.
The three ideas that explain almost everything
1. A workspace is the container
Every lead, project, invoice, asset and file belongs to a workspace. The workspace you have switched into scopes everything you can see, so the same page shows different data in each one.
One login can belong to several workspaces. You switch between them from the header.
2. Apps decide what exists
A workspace has a set of Apps switched on. Those Apps decide which menus exist at all.
"Why can I not see the Projects menu" is nearly always "that App is not on for this workspace", not a permission problem and not a fault.
3. Roles decide what you can do
Being in a workspace gets you in the door. Your role decides what you can open once inside. Somebody with no role assigned is a member of a workspace they can barely use.
Those two gates are independent, and confusing them is the single most common support question on the platform.