OBJ doesn't support attributes so it will all get lost unfortunately.

So, can I know how FME converts IFC to OBJ?

It depends on who the converter is designed, however, the most simple would be to read the IFC File, and then just write it out to OBJ. There are a lot of different configuration options when reading ICF, If you choose to read everything then you will get allgeometries. FME will create internal FME geometries out of the IFC and then on the way out to OBJ it will do it's best to convert these geometries into OBJ mesh/lines geometries.

You can do a lot more in your conversion, for example you could output a single obj file for each ICF type. The filename could be based on the type name for example.