You can remove the brush very easily and then also clean the water supply and drainage channels very easily. I had the Kärcher hard floor cleaner before and it's really bad there. You can of course add any agent, but of course Bissel says that only theirs is "right" in terms of cleaning performance and possible clogging of the channels.
I usually only use hot water. Sometimes I also use another, non-foaming agent.
As a tip, you can get replacement materials such as brushes etc. on AliExpress for no money at all 😀