I found an iPhone S the screen was broken and I fixed it, I found it in 2020 but I couldn't unlock it. I want to put it in use. what should do with the phone to unlock it?

If the phone is under an Activation Lock – because its owner marked it as Lost, using Find My – you don't. You can only remove an Activation Lock if you are the phone's owner. (This means that many stolen phones can be "bricked" by their owners, making it less attractive for thieves to steal iPhones in the first place.)

Activation Lock for iPhone, iPad, and iPod touch - Apple Support

How to remove Activation Lock - Apple Support

You can't. It doesn't belong to you.

Activation lock is an anti-theft system. Recycle it responsibly or turn it over to the authorities as you should have done when you found the phone to begin with.

You will need the Apple ID and Password that was originally used to set up the phone in order to unlock the phone.

The phone is useless without this information.

Bob Timmons wrote:

You can only remove an Activation Lock if you are the phone's owner.

You don't have to be the owner, you just need to know the Apple ID and Password that the owner used.

You're right. So if you're the owner, that's an excellent reason to use a reasonably-strong password, and not to disclose it to anyone except someone who is trustworthy and has a need to know (e.g., a spouse).

Certainly you wouldn't want to make the password available to any random stranger who might find the phone – or worse, to a thief.

You can only remove an Activation Lock if you are the phone's owner.

You don't have to be the owner, you just need to know the Apple ID and Password that the owner used.

In most cases, the owner does not even have to have it marked as lost. As long as Find My was turned on, the device will be locked to the owner preventing it from making any changes until the original owner has removed the device from their Apple ID.