JEE

You can expect some lower NITs with lower branches also you have good chances of getting decent IITs with good branches so don't give up and participate actively in JoSAA.

With these marks it will be almost impossible to get admission in the college.

The percentile which you got is very less as compare the percentile required to get admission in IIT and NIT.

Yes you will be able to get admission in UPSEE college for most courses.

SRMJEEE Rank holders of 1 to 100 - 100% waiver of Tuition fee, Hostel Fee/Transport Fee
SRMJEEE Rank holders of 101 to 500 - 100% Tuition fee waiver
Children of Defence Personnel - 25% Tuition fee waiver.

No u may attend special round, once try your luck.

May be you have a chance to get in government college depending on cut off. But not in popular government college. It is quite difficult to tell you about the cut off as it depends on various factors like: Number of aspirants appearing for the exam, Number of aspirants applying for the exam, Number of aspirants qualifying the exam, Difficulty level of the paper, Previous year cut off trend
with a sc category rank of 20,190 you do have chances in government colleges but you won't get into top branches or top colleges. You would have lower and newer NITs, IIITs, and GFTIs.

A candidate who has passed Qualifying Examination I. e. any Diploma examination or equivalent examination and obtained an aggregate minimum of 45 % ( for General Merit Candidates) of marks taken together in all the subjects of the final year (I. e. , fifth and six semesters) Diploma examination (Q E) is eligible for.

I would say initially get mug the NCERT Books completely.

No, you can not get admission into any college with your JEE Main rank, even under the Schedule Caste category without a home state quota. However, this is just an estimate based on the previous year's cutoff, and if seats are available for this year, you may also get admission at the National Institute of Technology and Indian Institutes of Information Technology. It also depends on domicile and gender. For more details, you may the Shiksha JEE Main college predictor. (https://www.shiksha.com/engineering/jee-main-college-predictor).