What happens next ?

If termination is allowed which is by no means guaranteed your case will go to the I-485 unit to schedule an interview. That can take many months....

How can I obtain a birth certificate?

What country were you born in? If you pass, YOU will never be able to get a death certificate but your surviving family may.

Will i get a visa after returning to my native country after living in the US illegally for nearly 28 years?

You will not likely ever get a tourist visa. Depending on the particulars you may be able to get an Immigrant Visa down the road.

Would I be able to sponsor my niece here in USA if I am a US Citizen?

Only if you sponsor her parent who is your blood relative and your niece is under 21 when blood relative gets immigrant visa

You are considered to be in the immediate relative category. It is best if your paperwork is properly filed by 3/31 but you wont have an issue even...

I agree with my colleagie. If you respond adequately the case can be apporved.

With extension filed you can stay until decision or time requested expires unless prior to extension expiring you file a second extension.

What about the undocument immigrants?will be able to apply for being legal according to Biden?

No one knows yet and more is needed about your boyfriend’s prior marriage and the paperwork he filed with Immigration thru her

Did you file married filing separately and list the same address on each? Single? Jead of household?

After biometrics how.long it takes to get naturalization interview??

Likely several months - USCIS is backed up die to Covid.