Sharing a folder with external users - report and best practice

Hi,

I am doing  revision of way our company share folders/files on SharePoint with external users/partners... and trying to get some advise on report and also best practice 

1.  report
   • I ran report on Documents from Sharepoint via Site Usage -> "Shared With External Users" section... but it is a bit messy.
   • I try to filter the report based on two attributes
     • UserName - "ShareLink"
       • using this filter, I should see all data that are shared via "link". However, there is huge amount of files/folders that -when I check- do not have any shared link created at all in "Manage Access"
     • User or Group Type - "External"
       • this seems to work fine and lists all files/folder where external users are visible under "People" tab in "Manage Access"
2. best practice
   • what would be the best practice for secure sharing folders/files with external users for short-term? 
   • what would be the best practice for secure sharing folders/files with external users for long-term?

I am also still a bit confused about the 3 possible ways of sharing... Just can't find some simple explanation on what are the differences between the 3 below:

Looks like the company was not very consistent in sharing externally so looking to set up      some basic rules :)...  and also to share with external with a way that is easily trackable - via the export mentioned above for example...

thank you in advance