When DC were that age, I read Positive Parenting by Jane Nelson, did what she suggested and the tantrums disappeared overnight. Never returned, not even in teen years. It was over 20 years ago but I bet the advice still works.
The things that helped me most were:
Agree with how they feel, not with what they want. So, tantrum about carseat, you say, 'I know. You hate it! Horrible car seat. I wish you didn't have to sit in it. But it's for safety, and if police saw you without one, mummy would get into real trouble, so let's get upu in quickly, get home safely and then you can cuddle the cat/ watch CBeebies. '
If it's a really tricky situation I would ask for 'suggestions ' from the toddler, and 'discuss' them. It is quite a talkie solution, but it beats gritting teeth while they scream.
At home if they melt down, create a cosy corner with cushions, blanket, cuddlies, a favourite book, a healthy snack. Sound sympathetic, not angry, and take them there and say, 'Snuggle up here and cuddle teddy until you feel ready to talk to me without shouting/ play without throwing things' etc. This is WAY more effective than punishment as it teaches them responsibility for self soothing and controlling their own emotions.
Also, try to plan the day. Tell them what is happening at the start of each day and recap often. They like knowing and feeling in control. Give choices ( all of which suit you) where possible. Save treats as a way of getting them to behave. E.g. instead of saying, we have to leave the park now, you say, 'Time to go home for a biscuit.' Generally, keep healthy snacks and water to hand, and ensure their clothes are comfortable, not too tight or hot, that they have somewhere to nap if tired. Most tantrums are triggered by physical discomfort or confusion about what's happening.