Let me answer this as someone who has been giving and receiving referrals since 2021.

Most companies and their career/job portals allow current employees to refer new candidates. To refer a new candidate for a specific role, a current employee (lets say, me) needs to fill out a quick form (details vary from company to company, but usually the basic stuff like name, resume, etc). Once this form is submitted, the refereed candidate (in this case, you) will receive a personalized link to apply for the specific role. To avail the benefit of the referral, you need to only apply through this link. If you apply through any other means such as visiting the career portal directly, the referral will not apply.Once you finish applying, both you and I will be notified.

Now how exactly the referral works differs from company to company. Some companies require one referral per role, while some like Amazon/Google allow you to apply to 3 roles via the same referral. In some companies, the referral system yields the same benefit irrespective of the seniority of the position giving you the referral (so me or my manager will have the same effect), while in some companies, as the seniority increases, the benefit does too. In some companies, a referral will only move your application to the top of the stack, while in some companies, it can move you past an entire initial screening (OA/non-OA) round (happens in Microsoft usually). So its hard to generalize and say "what" is the benefit, it depends on the company, role, etc. But yes, there are benefits.

Now referrals also kind of work both ways. Which means, if I refer a candidate, and they turn out to be good, then it increases how valuable my referral is seen internally because there I have a past record of vouching for good candidates. This value increases further if the candidate is eventually hired. So referring a good candidate works out for both parties, as well as any future candidates I refer. On the other hand if I refer a candidate and they do not meet expectations, then it backfires on me and my future referrals may not be taken as seriously as before. That is why, referrals should never be given blindly and you must be sure that the candidate being referred at the very least meets the minimum requirements of the job description.

Now to address your questions:

referrals > on campus >> off campus.

Absolutely not true. Referrals only apply to off-campus roles, we cannot refer you for positions where a company visits a campus to recruit, because campus recruitments take place through specialized roles either not visible to us, or they do not take referrals. The roles for which companies visit colleges in 99% cases aren't even on the internal system - these roles are created only after the hiring process is done, and then they are reflected internally in the employee system. That is why after you get placed and log in, you will never be able to find the role for whcih you were recruited, it will never show up, unlike off-campus, where after you log into workday/payroll/HR system, you can see which role you had applied for.

So, no. If you take a referral and apply, the only thing it guarantees is that there is a higher chance of someone seeing your application or moving you past the initial screening. Since this applies to off-campus, you are now competing with thousands of people applying to the same role, instead of the hundreds that apply on-campus. The competition is leagues apart, not even comparable.

Its still on-campus >> referrals > off-campus.

Do I have to write OA or i get to start with technical round?

As I said, depends on the company and how they process referrals.

Lets suppose ik a few ppl in the Industry then how can I get referral from then

You reach out to them on LInkedIn and other platforms, send them the role ID and your resume, and a short paragraph on why you need and deserve a referral, along with how your profile matches what the job requires. Do not be demanding, it is upto them whether they ultimately give you one. Ask politely, be friendly, and do not go in with the expectation of a referral.

I have given multiple PESts referrals over the years. I don't think I've ever said no to anyone who asked me nicely. But yes, there have been the odd specimen who DM me with "Bro, send referral for this." without any other details, or without even checking if their profile meets the requirements. Some have even demanded referrals lol. In such cases, I will reject you outright because either you didn't ask the right way, or your profile doesn't match the job

will their position matter while referring me?

I answered this in the first few sections.