Most of those ones you linked appear to be put on by the colleges themselves and just their staff. Sometimes smaller schools in a particular region do “pool” together for more “organic” multi school ID sessions. A top 20 DII program in our area does a 1 or 2 day ID session every summer, but they do have 4-6 other programs from the state involved and advertised on the camp website. I view those as similar to an individual school’s ID session, but be sure to contact any school of particular interest before hand to let them know you are coming and interested.
The camps that are put on with multiple colleges will generally advertise it in their e-mails and websites because for many it is a selling point. “Be seen by XXX number of colleges”. Many are “traveling shows” that hit multiple markets. The more regional/national events a player plays in, the more e-mail invites you get. I remember “Ryzer”, Future500, “Academic50”, Exact sports, etc.