I think the website actually answers these pretty well - it varies by program. The overal Q&A answer says the following (
https://www.montgomeryschoolsmd.org/uploadedFiles/curriculum/specialprograms/admissions/Immersion%20FAQs13.14(1).pdf):
7. What factors are considered in the lottery?
Depending on the program, there are approximately 26 or 52 seats available in the lottery for
students entering kindergarten. The available seats in the lottery for students entering Grades 1–5
are those seats vacated by students who withdraw from the program. Factors that may be
considered when running each school’s lottery include—
• the student’s high school cluster (eligibility);
• seats available for Grades 1–5; and
• the student’s older siblings in Grades K–4 who currently attend the immersion program
to which the child is applying.
All other assignments are made by a random lottery process.
Then if you look under Admission for each language, it describes the process:
https://www.montgomeryschoolsmd.org/curriculum/specialprograms/elementary/immersion-spanish.aspx
https://www.montgomeryschoolsmd.org/curriculum/specialprograms/elementary/immersion-chinese.aspx
https://www.montgomeryschoolsmd.org/curriculum/specialprograms/elementary/immersion-french.aspx