I have been using Git for a number of years and I can remember feeling quite daunted at the complexity of some of the commands I saw on the internet. When I started using Git on a daily basis I soon realised that the basics were quite simple and the complexity only lay further down the road with commands like cherry-pick or rebase.
When installing composer dependencies those dependencies are downloaded and stored in the 'vendor' directory.
If you have changed or updated a git repository and want to throw away your changes then a good way of forcing the latest changes to run a combination of two commands. The first command is git fetch --all, which tells git to download the latest updates from the remote without trying to merge or rebase anything.
The du (or 'disk usage') command is a Linux command that can print a list of the files within a directory including their sizes and even summarize this information. It is useful if you want to see how large a group of files is and provides more information about directories than the ls command does.