A variety of ways and it varies across species. Some animal young purposefully eat adult feces to seed their guts. Others pick up a sampling during birth from o

A variety of ways and it varies across species. Some animal young purposefully eat adult feces to seed their guts. Others pick up a sampling during birth from obvious sources. Also none of us wash our hands religiously enough to avoid 100% fecal contamination so babies pick bacteria up indirectly as well. Some people theorize that's behind their "mouthing" behavior.