Re the internships – They will be considered a very good EC/summer activity. If you have done interesting work, taking on increasing responsibilities etc. that is a positive as well. It also shows some commitment to what I assume is an area of interest as well. But they will not get your into a college you are not academically qualified to attend.
Re the letter - If anyone has a supplemental letter of recommendation from a boss testifying to personal/intellectual/work ethic etc. attributes that is a positive as well – but recognize that it is what the letter says that is important not who wrote it (a great letter from a boss at McDonalds can also be a positive in an application).
Many applicants to these top colleges have done a wide array of impressive things. There is no predicting what will stand out to any one admissions officer at any one college on any one day. Good luck.