a couple of things here:
1/ yoshi is testing boundaries. i find all birds go through this, they can be sweet, then decide they're going to go you if you do something they dont like. my musk lorikeet was really sweet the first few weeks, would stand on my chest, put his head down like a dog and want to be patted. a few weeks of this then he decided he'd bite me if i tried to get him to step up and he didnt want me to.
2/ people will disagree, but i'll tell the bird to stop it when they're biting or something like "oi". this can be seen as a way of reinforcing, but if paired with an unpleasant thing for the bird (such as being put down, or being put back into the cage) then the bird can learn that when you say it, it's time to stop what they're doing
3/ there will times yoshi will be doing something and you'll try and take him away, hence the bite. in these cases unless you really need to move him (time to go back into cage, a dangerous situation etc) i'd leave him be
4. learned helplessness whilst conceptually is bad, can actually be quite helpful (i expect a lot of disagreement on this). the ideal is your bird wants to do something i.e. step up, and you can motivate or encourage that behaviour. there are times this just doesnt happen or theres an urgency which leaves you to "force" the behaviour you want. take yoshi, you really need to get him into his cage for whatever reason. you can ask him a dozen tiems to step up and all he wants to do is bite/try and bite you. eventually you can try and lure him to step up, or you can move your finger closer to him, cop the bite and have him step up. the idea is eventually he learns he's gonna end up on your finger anyway and he'll then hopefully realise that fighting with you over this isnt worth it. i have several birds i've had to do this to and they now are used to getting onto my hand without any fuss. it's not the first option i'd try, but as a last resort can work well. there'll be the argument that you'll end up scaring your bird or making things worse, but if you've developed a good bond with your bird this shouldnt be an issue