Hey Nigel,
We'll get you thru, but there will be some points where it may be worth watching videos or grabbing a paper copy of Mac for Windows Users. Let's start with some of the basics. What version of MacOS are you running - you can find this by going to the Apple icon in the top right and going to 'About this Mac' - it'll give you the name, and below it a version number like 10.13.6 or such.
There is a folder structure you can see, it's just a matter of if the left sidebar shows them. Click an open area of the desktop, and in the top left next to the Apple icon, it should say 'Finder'. Click 'Finder' and then click 'Preferences'. In the window that pops up, you want to click 'Sidebar' at the top. In the items here you want to check - make sure the 'house' icon is checked. This is your user profile directory. It'll have the 'Pictures' and 'Documents' folders you're expecting from the PC world.
As to SSD's - yes, they're much faster than spinning rust drives, even over USB2. The SSD's aren't at full speed on USB3, but USB3.1 and TB (1/2/3) connected drives are even faster.
-Joe