Ah Harold, that's why you superglue the lid shut
OP it helps to think less about "tantrums and defiance" and more about what kind of morals you want your DC to have when they're adults. Obedience is for dogs, not for small humans who will one day be big humans.
Tantrums especially the under-3 kind aren't about being naughty, they are just really overwhelming feelings that the child doesn't know how to express or control yet. Your job is to minimise damage and prevent them hurting themselves or others, and when they've calmed down (or in advance if you're quick enough to spot the signs and lucky to have a child who doesn't go from 0-60 decibels in about 3 seconds) then it's important to help them control and express their feelings in a less explosive way. This is a long process, you can't avoid tantrums completely, the best thing you can do is try and have empathy for them.
If you can imagine being 18 months or two years old, your worldview is pretty limited and most things are still new and interesting to you. If you picked up something interesting and were looking at it to see what it is and what you can do to/with it, maybe tasting it or squashing it to see what happens. Suddenly a grown up comes over, shouts something and takes it away from you. You'd likely feel shock, a little bit of fear, anger that they've taken it off you when it was yours, sad that it's gone and you can't play with it any more. You don't know if they're going to give it back (and you have to assume not since sometimes you don't get stuff back) and your only way to communicate wanting something back is to point and say "Want that" or something else vague and blunt. It's not like you can form a reasoned argument for wanting it or enter into a discussion. Plus you're rather emotionally immature and unstable anyway, so something that hits off this many buttons at once is likely to provoke a big reaction, tears, screaming, whining, even physical expression like stamping or lying on the floor.
Obviously that doesn't mean that if you see your toddler picking up something dangerous or potentially harmful or a live animal or something that you shouldn't take it off them, but you can be sympathetic as you do so, you can understand that it's quite a big deal to have something taken away, you can try in the first instance asking for them to give it to you calmly rather than panicking and grabbing it off them (which often makes it more desirable anyway). You can show them the dangerous part in a safe way, e.g. by demonstrating that touching the sharp part of a knife is "ouch!" And most tantrums are similar - they don't understand the reason behind something and they're too little to have it explained, the situation is too intense for them to cope with, they are feeling too many things at once. Tiredness and hunger can contribute too. And they haven't developed the skill of perspective yet. A splinter is as bad as breaking a bone. The biscuit being broken is as important as your favourite toy being beyond repair. Losing a balloon is like losing your best friend in the whole world. They just can't differentiate between something being a little bit annoying and something being the end of the world.
Defiance is all linked up with obedience and it's better to think not "He should do as he's told!" but "Why do I want him to do X" - maybe you want him to pick up his toys because you want him to be a tidy person and not leave mess for everyone else. Maybe you want him to apologise for something to help him learn emotional responsibility and kindness, maybe you need him to obey instructions for safety reasons. Whichever it is, there's always a different way of handling it and a different way of looking at the refusal. For example, for safety issues there is no leeway and you either decide that he's too young to do that activity reliably/safely or put measures in place so that it is safe (pushchair/reins near roads, for example). At a certain age they are able to understand a pre-warning that this activity is dangerous and they need to listen straight away or they will be stopped from doing it. For other things, like the apology you can keep modelling it and showing him it's the right thing to do, he won't get it straight away and it might take longer for the message to appear "accepted", but when it is accepted in this way, it's more likely to be true to what you actually want rather than him learning it's a meaningless thing to say when you want to get out of trouble. With tidiness, again, model, involve him in general tidying stuff, make sure he sees you and DH cleaning up and doing their bit and not just you running around after everyone, make it an everyday thing to clean up after yourself and he will eventually find it second nature - but he still probably won't want to tidy up his own toys when presented with that as an instruction, perhaps because it feels overwhelming or he doesn't want to do it alone or he doesn't want to stop what he's doing now to do that - warning in advance can help and also helping him yourself. Although again this is one of the long games, where you are unlikely to see results earlier on but long term if he encompasses cleaning and tidying into "just what you do" he'll develop more of a clean and tidy personality and do it off his own back, rather than doing it because he's being nagged but not really seeing the point of it.
Sorry that was long I hope some of it is helpful.