Export vs. ConvertTo - what's the difference?

I am a beginner to PS and am enjoying the book by Jones & Hicks (am now past Ch. 8), especially the exercises (“LABS”).
I have noted to myself that while there are “twin” cmdlets such as: ConvertTo-CSV / ConvertFrom-CSV,
this “duality” does not exist for: ConvertTo-HTML, ConvertTo-XML – meaning 'To’s exist but there are none for 'From’s.
I think I know why, given the meanings (or nature) of CSV versus HTML and XML files, but that is not the reason for my post.
I still do not fully understand the fundamental difference between:

  • ConvertTo-CSV versus Export-CSV,
  • Export-EXCEL versus Export-CSV << on this second pair, all I know is one is a cmdlet, the other a function.

the HELP and ABOUT files files don't seem to explain much.
Would be grateful for any tips or pointers or references to existing docs/websites that can explain these differences better.
Thanks in advance!

Export-* typically results in an export to a file in that format.

#Export services to a CSV file
Get-Service | Export-CSV -Path C:\Scripts\Services.csv -NoTypeInformation

Convert-* is just converting a PSObject into the requested format. For instance, you might take data and convert it to JSON to pass it to a web service\API.

#Get the first 3  services Name and Status and convert it JSON
PS C:\Users\Rob> Get-Service | Select Name, Status -First 3 | ConvertTo-Json
        "Name":  "AJRouter",
        "Status":  1
        "Name":  "ALG",
        "Status":  1
        "Name":  "AppIDSvc",
        "Status":  1

Much thanks, Mr. Rob Simmers.
Your inputs are very helpful to me.