Can you use hydrochloric acid or hydrogen peroxide to clean a toilet? Why or why not

If you used those "chemicals" what would happen?

hydrochloric acid may do the job, but it is extremely dangerous to work with so you should forget about that. Hydrogen peroxide however comes in different concentrations such as 3% all the way up to 35% (food grade) in commercial retail. Again, you can't use high concentrations such as 35% because this too is dangerous to handle. However low percentage (3%) H2O2 may be used for cleaning ceramics, etc.. After exposure to HP you should wash your hands.... and dont get it in your eyes... better to wear goggles ! The extra oxygen in the liquid is the cleaning and whitening agent.

Nothing would happen if you used peroxide (peroxide at the store is less than a 3% concentration).

Hydrochloric acid should not be used for cleaning because it is one of the "strong acids"; it dissociates almost completely. This could damage your pipes because metals become soluble in water in acidic conditions.

Store-bought bleach is extremely dilute sodium hypochlorate, so it's safer than HCl to use but it could still damage pipes and also damage septic if you use your house's septic tank (instead of city's).

Yes, you can. Acid will eventually mix up with water and lose concentration once it goes down the drain. Hydrogen peroxide is used by many cities to reduce the smell of waste water.

Using a commercial cleaner would be much cheaper, smell better and be safer in the case of acid.

I am not sure that it would be advisable, there is chemicals in the water in your toilet, such as chlorine.
I don't think these chemicals would react to the peroxide and hydrocloric acid. Might cause an explosion in the toilet