IIT

There are success and failure stories of JEE with or without external support. The choice of going for self-preparations or coaching for JEE depends upon your personality, intelligence level and budget. If you are self-determined student with high IQ, who is focused and capable of achieving targets based on self-assessment and hard work, then you can opt for self-preparation.
You can also join mock test series with coaching institutes along with self-studies, so that you can assess yourself with others and take corrective action to overcome your weakness.

You can get admission in IIT by cracking JEE Mains + Advanced.

No, as the pattern is changed, but formulas and concept is same.

Hello Sonali Patil,
Diploma holders in engineering are not eligible for admission in B.Tech as IITs do not have provision of lateral entry admission in B.Tech.

NIT Surathkal, Gandhinagar, Kurukshetra are the best. You can also go for VJTI Mumbai, Sardar Patel Mumbai, BITS Pilani, ISM Dhanbad as they offer best placements into MNCs and conglomerates.

You can check the below mentioned link:
https://www.shiksha.com/b-tech/articles/best-books-for-jee-main-2020-preparation-blogId-19529

Hello Debargha Samanta,
Keep these points in mind before starting preparation for IIT:
Time management
Follow a schedule / prepare a plan.
Know the syllabus and the exam pattern well, weightage.
Coaching classes online or offline? Self-study.
Study concepts, solve problems and look at solutions.
Solve previous year's papers.
Take timed practice test series.
Group study, hangout more with toppers
Health is wealth.
Manage Distractions through self-control app.
Retrospect your day.

Hello Aman,
If you are clear about your career that you want to be in IIT then go for it. You can drop and start preparation but don't spoil one if it's not serious for you.

IITs and NITs do not offer degree course in BCA.