My husband and I separated but had a marketplace health plan jointly. How do we file our taxes accurately but separately to reflect this?

You need to get a copy of the form 1095-A and enter it in TurboTax in the Deductions and Credits section, then Medical, then Affordable Care Act (Form 1095-A). You will be asked if you shared the policy with someone and you will be given the opportunity to indicate what portion of the policy you paid versus your spouse. This is assuming you file as married-separate or Head of Household (if you qualify.)

Your spouse would then do the same on their return. If you are still married you could also file as married-joint and then just make one entry for the health insurance premiums.

separated or legally separated at the end of 2021

it matters

see the instructions starting on page 16 for line 29 of Form 8962 instructions

situation 1 only applies if you were divorced or legally separated that occurred in 2021

situation 2 applies if you were not legally separated

https://www.irs.gov/pub/irs-pdf/i8962.pdf