Hello !
Blockbuster, to assess OH&S risks from identified hazards, "while taking into account the effectiveness of existing controls" means to assess the "current" risks, as assessed by today's reality of work situations "with" or "after" the prevention dispenses already in place and practiced... In other words, it is a question of assessing the real risks and not the "initial", "raw" or "theoretical" risks "without" or "before" the effectiveness of existing controls.
I try a trivial ilustration: I drive my car according to the regulation, it is consistent and in good condition, I drive on a highway in great weather, there is little traffic, I did not drink alcohol, etc. ... I will not start my assessment of risk of accident considering that my car has no brake and that its wheels are smooth, that I roll on a dirt track at too high speed, the weather conditions are bad and I am drunk, etc... to then "apply" fictional the goods points of the reality !
Another simpler but equally trivial example: I'm standing on a balcony on the tenth floor of a building to enjoy the view. I will directly consider (evaluate) that the risk of falling from the balcony is zero because there is a railing! That is to say, I will not evaluate this situation at first by considering the risk of crashing to the ground as if there were no railings (stupid gross risk!) ... to "apply" in a second time the already existing "guardrail" prevention measure and finally consider that I don't risk to fall into the void (current non-risk directly evaluable, otherwise I think I would not put myself in such situation on this balcony).
Other: if I want to measure which quantity of dangerous vapors I breathe (risk assessment) when I am on a workstation equipped with a vapor collection system. I will not stopping this aspiration (= not taking into account the effectiveness of existing controls) before ! I will do my measurement "while taking into account the effectiveness of existing controls" as said in ISO 45001 = I keep the vapor collection system running !
Ok ?