The nature of work for both sectors is research based. Work at ISRO is multidisciplinary which means you can be doing whatever is required to get the final product. And your job role will depend on the Space Centre you have been assigned to. There are inter departmental transfers and so if you joined initially in Physics domain, you can move to Electronics domain and vice versa.

After completing B.Sc Physics, you can join ISRO as their Technical Assistant. With experience and upgrading your Academic qualification, you can move up the ladder. But the opportunities are lesser for Physics students when compared to Electronics Students at ISRO. As someone from Electronics background, you may have to support a Mechanical Engineer's work or co-ordinate work between various agencies. At times you might be also analysing data from payloads for satellites.