1) Will need to file separate forms application when applying for citizenship? More question below:-

Yes you may wait until becoming a USC to apply for your children and sibling.

Do I (the petitioner) have to be present in the US while the visa application for my son is being processed in the Philippines?

Yes you must have domicile in the US.

My us citizen baby born with a medical needs as of now not yet fix so we need to stay but our cw1 gonna expired

You should apply for an extension of your visa based on your children’s health concerns.

Can I buy a car with a B1 visa in Indiana?

For driver license concerns it is best to consult with the DMV directly.

What can I do to expedite the reschedule of my interview?

You should follow the instructions of the officer and write to USCIS regarding your request.

IIf I file my asylum case in San Francisco.and after filing the case can I go to Florida I know some people in Florida ?

Yes you may relocate but should notify the asylum office in writing of this move.

No your children will not be taken away for not working under the H1B.

No this should not detrimentally impact your mothers immigrant petition. Consult with an attorney for guidance re: filing.

A letter of this nature is not required and the sponsor can draft one in his/her own words.

I am filing an i-864 for my spouse but don't have the 2018 tax return, I didn't work. i only have 2017 and 2016 !

You should indicate on the I864 that you were not required to submit a tax return and then use a co-signer to fulfill the affidavit of support...