NIT

Yes, you are eligible to register for JoSAA counselling even if you are not eligible for JEE Advanced. Your 31 percentile and 30,000 rank in ST category allow you to participate in JoSAA, although the chances of getting top NITs may be low. You may have a better chance in NITs from the North-East or in less competitive branches. Make sure to list all available options during choice filling and consider state-level options too.

With an OBC NCL category rank of 1,51,000, it becomes quite challenging to secure a seat in the core branches of NITs or IIITs through JoSAA rounds. However, a few newer IIITs or GFTIs may still have openings in streams like Metallurgy, Production, or even Civil in later rounds. Keep an eye on spot round announcements and also consider private colleges with good reputations and lateral entry or transfer options in the second year.

Since you have a 98.83 percentile in JEE Main, there are great chances of computer Science (CS) branch in various good NITs. Although you may find it quite competitive to secure CS in the best NITs like the Trichy, Warangal and Surathkal, chances are you will get CS in NIT Delhi, NIT Kurukshetra, NIT Durgapur, NIT Jalandhar, NIT Patna, NIT Raipur, NIT Hamirpur, NIT Goa, NIT Silchar and NIT Andhra Pradesh. Your prospects are better still, when you have a home-state quota or fall under one of the reserved categories. Make sure that the cut offs of the previous year are looked up in JoSAA counselling.

With a JEE Main percentile of ~83, your All India Rank is likely around 150,000–200,000. Unfortunately, that's well above the closing ranks for B.Tech Biotechnology at NITs, which typically cap around 30,000–50,000 .So, no NIT is available for Biotechnology with your rank. Mechanical also would be out of reach. These branches close much earlier in counselling rounds.

You can study a 5-month online Professional Certificate Course in Data Engineering offered by IIT Kanpur after completing your graduation in a related field. For the NITs, none of them offer a dedicated Data Engineering course as yet, but you can have a look at related courses like BTech in AI and Data Science or BTech CSE with some elective modules on Data Engineering basics.