Hey Selena,
first of all, thank you for the purchase!
I will cover the questions in the same order:
1). A “slug” is another name for the human-readable part of a web page’s URL; for example, if your website’s homepage URL address is “example.com”, then a contacts page on that website may have a URL of “example.com/?p=34”, which does not convey a lot of information to the end user – or it can be “example.com/contacts”, which is a semantic URL with a slug “contacts”. You can read more about URL slugs and permalinks here and here.
2). The code you’ve provided needs to replace line 69 of the “page_portfolio.php” template file, yet if you are not familiar with PHP, I would suggest that I make the changes for you, because a single PHP code mistake may render your website quite unusable. If you’re using an FTP client, I can provide you with a changed file – or you can give me access to your WordPress backend, and I will implement the code; as you wish.
3). This is not possible in WordPress without using a separate plugin, e.g. this one – otherwise, the “tag-portfolio” part is the slug of the portfolio tag, which cannot normally be avoided.
Sincerely,
Andrii / Satori Studio