If I start a STEM application now and also file for green card with I485/I-30 forms before Feb, is my STEM application void?

Immediately upon filing of an Adjustment of Status based upon marriage to a U.S. citizen, your status will become "Applicant for Permanent Residence" and you lawfully may remain present in the U.S. That will be true regardless of whether the application is filed before or after your current OPT expires. The USCIS will issue an Employment Authorization Document ("EAD" or "work permit") within 60-90 days of filing, authorizing employment while your Adjustment of Status application remains pending. Filing an application to extend F1/OPT, and the timing of the filing, can presents complications, since that status is only for people who intend to remain temporarily in the U.S. for the purposes contemplated in the OPT application, and then depart. (This same complication does not exist for someone with an H1B application, since that type of visa recognizes "dual intent" - i.e. an intention to remain temporarily in compliance with the H1B and also an intention to remain permanently if that person becomes eligible to do so.) Both for achieving your long-term goals and for eliminating or minimizing any gap in your authorization for employment, it would be wise to engage an immigration attorney to learn all of the relevant details about your goals, your immigration-related history, your job, etc. in order to advise you about eligibilities, options and strategies.