ISRO releases invitation for application depending upon their vacancies. But yeah! Every year some amount of Vacancies always released by ICRB for Engineers from different disciplines as well as for Architects.

You can get the syllabus of ISRO from the ICRB notification when you will appear for the examination.

For ICRB portal you can visit:

https://www.isro.gov.in/careers

I hope this answer helps. All the very best for your future endeavors!