Hello.

1)Medical seats across all colleges within and outside your state are divided into 85% state quota(SQ) and 15% all india quota(AIQ) seats respectively
You get a state rank and an all india rank in neet via which you can seek admission to colleges in your state(SQ) and across other states(AIQ) - unless your state takes a separate pmt exam for state quota seats (like WBJEE , MaharashtraPMT , etc)..Supreme court now lays down NEET as the only exam for filling up all SQ and AIQ seats barring few states...
2)Depending on whether you want to use the SQ rank or the AIQ rank..
If your AIQ is 2000 and your state rank is 500..You might want to choose a seat in some other state's good college using your AIQ than a subordinate state medical college using your state rank in SQ..choice is yours..the reverse can also be applied using your rank is your state rank is relatively better and the college you are getting via state quota is better than he other colleges within or outside your state you re getting via AIQ
3)Next is simple..you have to sit for an online filling of choices..which will be notified on the site for neet..or your state medical board..you will be notified about it through internet..and see which college you are alloted to in 1st round..You have to go for a verification of marksheet and other documents in the alloted centre following which you will go to your alloted college and complete the admission process
4)You have a choice of altering between state and all india quotas..and whichever you us to get your final seat in subsequent rounds of choice filling and counselling..the other automatically gets nullified..If you are getting IPGMER via state and VMMS Delhi via AIQ..if you avail one college..other gets cancelled..
You can keep changing choices on the counselling platform(online..last round may be offline..you will be notified) unless you lock a college , get admitted in a college of your choice as per your rank and then enjoy your medical life there...
PS:Verification process takes place only once..but each time you opt for and are alloted a college..you have to collecet your documents from the last college and submit it to the new one..and get admitted to the new college after geting the leave letter from the previous college.

Hope this is helpful.

Good luck.