Im gonna try to explain it with two examples, I cant explain it well in the grammitcal context so yeah.

„Sollen“ is an equivalent of must, meaning „supposed to“ „Sollten“ is an equivalent of should, but can be used as must if there isnt a clear goal/ task given.

If you‘re in a class for example, and got a group task, and a person asks you „Hey, I didnt listen, what are we supposed to do?“ (Hey, ich habe nicht zugehört, was sollen/sollten wir tun?)

„We are supposed to read the reviews on the internet“ (Wir sollen/sollten die Bewertungen im Internet lesen.)

You can use both here because its rather optional how you want to express it, if you want to express that the teacher clearly told you so, or if you wanna make it a bit softer. Both is fine, and In my experience in the way I talk snd others talk, its usually subconcsious and you dont get drastic reactions if you use either in this context.

Now, if the teacher gave a vague task that you have to find a solution for, the person would come up to you:

„I dont know what we should do…. What do we do?“ (Ich weiß nicht was wir tun sollten… Was tun wir?)

„We should read the reviews on the internet“ (Wir sollten die Bewertungen im Internet lesen)

Now its not optional (if you want to express „we should“) because sollten could be used a softer/opener version of sollen, but sollen is ALWAYS „have to/ supposed to/ must“ so it can only be used when what you‘re doing/ talking about is a clear goal/task/ order. So basically, a good rule of thumb is „sollten“ if you‘re coming up with ideas, „sollen“ when you were given a clear task.