When you start filling in the gaps by saying "God did this, God did that" you're outside the realm of science, which is what this thread is about.
Once onboard, many could have been very small as even the largest species have tiny, humble beginnings as young.
Like elephants? Horses? Bears? Etc?
Many creatures may have hibernated on the journey and required very little if any care.
Most animals don't hibernate. Why would they start on the ark? How were they fed? Where was all that food kept? What was done with their waste?