As far as I know the Free web version has almost if not quite all of the features of the current Pro version.
It doesn’t support extensions at all, and may never do so.
So long as you are not (or not yet, anyway) using it for commercial purposes I think you would find it easier to learn on the last version of SU for desktop called Sketchup Make (v2017 was the last such version).
You can download it here for Windows or Mac, 64 bit only.
That version starts as a 30-day trial of the Pro 2017 equivalent, then reverts to Make.
The main things you don’t get in Make are some professional import and export file types, nor the Solid Tools.
But it does support plugins.
If you find it does meet your needs, and you want to use it for commercial purposes, then you would need one of the SU2020 paid for versions.
There are several topics in this forum on the cost and other differences between the current licence types which allow you to use SU for commercial work. They include different combinations of related programs, and some are annual subscription, while the Classic licence - a fixed price which allows you to use the software indefinitely - includes mainly Sketchup and the associated 2-D presentation software Layout, and a tool to allow you to design different line styles. Included in the purchase price for the Classic licence is the first year cost of an annual maintenance fee, which allows you to upgrade to the next later version.