How do I fix cracks in my grout?

Your bathroom should not be showing cracks like this. Either the underlayment or the grout was done incorrectly.

Did you get a warranty? I'd call the installer. If they refuse to remedy the issue, call your State's Attorney general's office, supplying the installer's license number, location, etc. A negative post on social media may further prompt your installer to recify the situation.

I can't tell from the photo if this is sanded grout or if it was sealed.

Epoxt grout is much more expensive, but worth every penny.

Grout is not used where the tub and wall meet. That is what 100% silicone caulk is for. Also used in corners. You will need to remove the grout and use silicone caulk. Have whoever installed the tile correct the problem. Or you can carefully chip the grout out with a razor knife or flat screwdriver. You can also use a hand grout saw.

https://www.homedepot.com/p/M-D-Building-Products-Heavy-Duty-Tile-Grout-Saw-49066/205329337?cm_mmc=Shopping%7CG%7CVF%7CD24%7C3M%7CNA%7CPLA%7C3M_Sundries_Paint_Test%7c71700000047523376%7c58700004766740015%7c92700040663644252&gclid=EAIaIQobChMI-Lnv5Z-R4AIVErvsCh1A7g5lEAQYAiABEgKbOfD_BwE&gclsrc=aw.ds

I agree with William. You need to have silicone caulk installed at that joint instead.