Hi john
Welcome to the wonderful, frustrating, fun world of growing mushrooms
The agar question is a helluva lot to unpack. There are a bazillion recipes for 'agar' ( which is really only a generic word for whatever gelling agent is used to solidify the semisolid stuff in a petri dish, but the common use now includes whatever else is in the mix ). Confusing as fuck IMO
Most people just use what's convenient for them that works, hence all the different agar types. Some types are specific to species, some are general, some used to deter competitors to get a cleaner culture etc
Are you using only agar and water- your OP refers to straight agar- or is there other stuff in there, malt extract, dog food, vegemite, lime etc
H2O2 ( hydrogen peroxide ) kills spore of bacteria and fungi without killing most mycelia- up to a point ( it can be overloaded, degrades if you keep the agar plates for too long, and shouldn't be added until your agar has cooled to the point where you can hold the container by hand )
3% hydrogen peroxide is usually added 2-8ml/L when your agar has cooled and is ready for pouring. It can negate the requirement for pressure cooking your agar and is also handy if you don't have a sterile cabinet or still air box to work in. I've poured H2O2 agar in the open in my kitchen and had good success with it. I usually carry some plates of it with me if I'm taking samples from mushrooms I find on bushwalks
You don't need H2O2 added to agar, but there are some situations which it really helps. Some species don't like it much and some take a while to establish on it
Hope that clears things up. Good luck!