Hello! 
Please, edit the post so you can also comment on what version of odoo you are on (v19, v18, etc) and what package (community/enterprise, on demand,.sh, online). Sometimes the solution for one version doesnt work with the other. 

On regards of the import contacts, depends on what you need. I believe there's no actual video/tutorial focused on your explicit need but this video maybe help you. 
What you need and want is a massive import of contacts. that is made by importing an .cvs or excel sheet onto Odoo and linking fields names, but then again. its somewhat complex depending on how many fields you want to import. 

Also, on the matter of making a massive listing mailing, I've never done it, BUT, if you have a field check for that, the above video will be helpful, you'd need to find the field's name and create a column with the same name and set it to true. 

Sorry If i am not very explicit on my explanation, then again, As i am not sure how much technical knowledge you have, I dont want to give you advice that could either be way too complex or that you may already figure out by experience. 

Best regards!