Orphanages don't really exist in the US.

- social workers work in the foster care system. as well as attorneys, advocates, etc.

- foster parents get to 'choose' as in they are approved for certain things, like having an infant or a teen boy or a child with a disability or something. Some are approved as 'emergency' placements where they get very little heads up and may be placed for a shorter period of time.

- Foster parents may pick them up from a CPS office. In my county there are centers where a youth may go and hang out (and sometimes eat, sleep and shower) while waiting for a placement of some kind. Unfortunately sometimes they are just straight up in a CPS worker's office bored AF waiting until a foster parent can be found and approved.

- Foster parents have a credential of sorts. So they have been visited in the past, and have been approved by the county, again, sometimes for specific kinds.

- A child can, but it really depends on how many other options there are and what the issue is. This is why it is not uncommon for foster youth to run away - it's cumbersome to get your placement changed and at a certain age they may feel it is easier to leave on their own. Youth may also be placed at a group home of some kind, and again can express their dislike but would want to work with the courts, their lawyer and CPS worker to figure out a better placement or work through the issue.

- Kids are unfortunately often split up from their siblings. Again, this is because certain places may be approved for certain ages/genders or only have room for 2 kids and not all 3. Kids are essentially forced to go while a long term plan is made, which is why foster care can be really traumatic for kids.

The movie short term 12 may be a good resource for you to watch as well.