Federal marketplace, not a state exchange
Some states run their own exchanges; Texas isn't one of them. Texas residents use the federally-facilitated marketplace. This doesn't limit your options — you get the full set of ACA plans and federal premium tax credits.
Three ways to enroll in Texas
You can enroll directly on HealthCare.gov, over the phone with the marketplace, or through a licensed agent or an approved Enhanced Direct Enrollment (EDE) platform. Using a licensed agent is free — you pay the same price either way.
What this means for subsidies
Because Texas uses the federal marketplace, federal subsidy rules apply. Your subsidy is based on household size and income and is applied the same way no matter which enrollment channel you use.