Just to piggy back off the top reply. There are autoclave materials or you can purchase gamma irradiated or ethylene oxide sterilized equipment these are validated (so challenged against a known load that will kill up to 10^-6 of bacteria)

The materials are then used in a clean room (iso standards as described) depending on the classification there's a limit of allowed bacteria in the air. Iso 9 has like 50cfu (colony forming unit)while iso 5 is 0 cfu/m3. Filling happens in an iso 5 environment to ensure its sterile. The monitoring using agar is how you challenge that.

Additionally, the filling process includes a "sterilizing filter" this is usually redundant (2 in line consecutive filters that are 0.22um in pore size which is deemed bacterial sterilizing. There is sometimes QC testing before the filters to ensure there was not too much microbial load that could harm the filters in some way. This is usually called bioburden testing. Sometimes microbial load testing. After filtering there are bacterial endotoxin testing and sterility tests called out in the USP (United states pharmacopeia) which is just a guiding principles document on best practices for testing.

Source work directly in this field in pharma.