How do spell slots work for druid/sorcerer in DnD 5e? [duplicate]

Please forgive the ignorance of this question. I’m new to DND and am DM with a group of lower level players. We are playing DnD 5e.

Please explain spell slots for a druid and/or sorcerer… am I correct to say:

Each day players "prepare" list of spells for that day, and place one in each spell slot to use later….

-is this correct?

-can each spell be used only once per day?

-do spells come from ALL spells for that level or only known spells?

-if it comes from known spells, how does a druid or sorcerer add to list of known spells?

For instance, I have a level 2 sorcerer who wants to use the same spell every round. We’ve played with the understanding that he can use that spell every time, but only as many times as spell slots he has. But, I think we are understanding this wrong.

Thanks for your help. I’ve read a ton but I am still confused on this.