"mailto:[...]" as you said, it is only a way to open the email client, and you can specify the following parameters:
mailto:[email protected] - specifies the e-mail destination
Subject - specifies the "subject" field of the email
body - specifies the main field of the email, is where you write your message
CC - Send a copy of the email to all specified email addresses
BCC - send a copy of the email to all these email recipients but without notifying the original destination of the message, it’s like you send a message on Whatsapp to X person, print the conversation, and send the print to people A, B, C, etc. People A, B, C will have a copy of the conversation without the person X knowing
? - this is to separate the domain used by "mailto" from the URL parameters, for example: https://qualquerdominio.com/uma-rota?parametro=valorDoParametro
& - This is to specify a second or third, or fourth parameter, for example:
https://qualquerdominio.com/umaRotaQualquer?parametro=valorDoParametro&outroParametro=valorDoOutroParametro
%20 is used instead of using a space, say I want to send a message with the subject "Good morning", rather than leaving this blank in the URL: "https://domioqualquer.com/umaRotaQualquer?subject=bom day", the URL is invalid, you need to replace the white space with %20, and then stay that way:
"https://domioqualquer.com/umaRotaQualquer?subject=bom%20dia"
I hope you understand, any questions you may have, just ask :D
I can already say that the two penultimate ones are not "parameters" - they are only parts of the URL. The latter, a space in encoded URL. The others seem to be self-explanatory...
– Luiz Felipe