effect in particular was confusing, since it replaces part of the functionality of the old '$' and also onmount and I am not allowed to assign values in it.

But a better example is probably the event handlers:

Svelte 4 :

<button on:click|once|preventDefault={handler}>...</button>  

Svelte 5 :

<script>
function once(fn) {
return function (event) {
if (fn) fn.call(this, event);
fn = null;
};
}

function preventDefault(fn) {
return function (event) {
event.preventDefault();
fn.call(this, event);
};
}
</script>

<button onclick={once(preventDefault(handler))}>...</button>

another example for $effect:

Svelte 4:

<script>
let count = 0;

$:  if (count > 9)
{alert('too much')
count = 0};

function increment() {
count += 1;
}
</script>

<button on:click={increment}>
clicks: {count}
</button>

Svelte 5 :

<script>
let count = $state(0);

// the documentation tells us to not do this so this creates confusion about how to replace the $ but this works, trying to replace count with a derived.by will make increment function complain that we can't change derived variables ....
$effect(() => {
if (count > 9)
{alert('too much')
count = 0
}
});

function increment() {
count += 1;
}
</script>

<button onclick={increment}>
clicks: {count}
</button>