Looking for tips and tricks for uploading CSV files

I understand the basics of CSV files, but I need to ask for clarification on a few things.

The Help file says “Importing CSVs does not overwrite existing database records, it only adds to them”.

Does this mean that if I first download a database and then add new records (or edit existing one), I should delete all the existing records first before uploading, to prevent duplicates?

Also, should there be a space after each comma delimiter?

How do I escape commas in the text that are not meant to function as delimiters?

Finally, can I use alternative delimiters, like tabs?