Recently had PRP where they spread it on the scalp and used a dermaroller over it, is this the correct method?

This is not ideal.  The dermaroller will not be able to get it into the correct layer for optimal results.  The PRP should be injected into the scalp.  In addition, microneedling can be used on top of that.  There is a big difference between microneedling and a dermaroller (microneedling can go a lot deeper).

I believe that injecting the PRP down around the hair follicles is more effective than using a dermaroller. The scalp skin is very thick and the dermaroller will only allow penetration a small distance into the skin and not down around the hair follicles. Every hair restoration surgeon varies though in their techniques. I recommend discussing your concerns with your hair restoration surgeon. Good luck!

The effective treatment is to take the Platelet Rich Plasma and injected into the scalp around the hair follicles. Microneedling with PRP is a technique than can be added on top of this procedure but by itself will not reach the desirable level.

Although dermaroller method is useful, it is not equivalent to PRP injections and is not as effective.  PRP injections yield higher concentration of platelet rich plasma and injected deeply into the thick scalp area. From years of experience and research, we use what is called Advanced PRP where we inject PRP directly into the scalp and then follow it with microneedling.
These treatments should be done by professional and experience doctor.

The injection of PRP into the scalp is the ideal method of preventing hair loss. While the dermaroller provides the microneedling effect, it is not equivalent to injections. The scalp skin is thick, making the dermaroller less effective in penetrating it sufficiently for the PRP to work. Good luck!

There is very little standardization when it comes to PRP. But what you are describing is microneedling with PRP; this is not the usual method for treating hair loss in the USA.  In my office, we inject the PRP fraction directly under the dermis in the scalp.  The use of microneedles is sometimes added additionally, but only after the deep injections are done. Be well!