What does "have fun" mean?

What's it mean by "here" here?

Here,"here" means "to this point," or this circumstance or situation, rather than physical location. Ex: I got here through hard work and motivation. = I got to this point in my life through hard work and motivation. Ex: I don't know how to get there. To a happy life. Ex: We never would have got here without you.

What’s the meaning of "milestone"?

A "milestone" is a celebration-worthy event or action that marks a significant achievement, success, or development. For example, we might consider some basic, common milestones for people to include graduation, marriage, and retirement. James Corden has used "milestone" to describe Ariana Grande's achievements in music, as the time she occupied numerous number one spots on the Billboard Charts. Ex: Thank you all for attending our wedding. We are delighted to have you here to celebrate this milestone. Ex: The Paris Agreement is considered to be a milestone for climate action cooperation.

Since it's several schools she's referring to, isn't she supposed to say "there are" instead of "there is"?

Absolutely! This is one of the most commonly-made grammatical mistakes by native speakers. The correct way of saying this is "there are" or "there're," but many people make the mistake of saying "there is" or "there's." You should also make note to avoid making this mistake when speaking. Ex: There's a bear in front of our car. (There is + single form noun) Ex: There are bears in front of our car. (There are + plural form noun)

Can I use "how" instead of "what" here?

Unfortunately no. You need to use "what" in this sentence because it would change the meaning of the question. When he asks her "what was the process like?" he is asking her to explain the steps of getting her wedding dress made. If he were to ask her "How was the process of getting your wedding dress made?" he would be asking her if she liked or disliked the steps taken to having her wedding dress made.

What is “bandar-log”?

The word "bandar-log" is not a commonly used word, almost only used in the Jungle Book series. It's a Hindi word, as 'bandar' means 'monkey' and 'log' means 'people; the term is generally used to describe monkeys.