When you create any folder, just click on the ellipsus besides folder (3 dots) -> shared with -> advanced settings. This is the page where you will find permissions on that particular folder.
By default it will be inherited from parent (library in this case). Click on "stop inheriting from parent". And now you can add/edit/remove permissions for that ...
Not with SharePoint groups. You can do this with Active Directory Security Groups. You would add users to the AD Sec Group then add the AD Sec Group to one or more site collections within SharePoint groups.
Just like Ganesh said, if you need to only share the particular list to the users. Break the permission inheritance for that list and share the list to the users.
And you can get more information about permission levels inheritance here. Check the "Overview and permissions inheritance part"
Understanding permission levels in SharePoint
If you want to add users to any particular SharePoint Online list only the you need to break permission inheritance on list and grant then grant the access to user on that particular list.
Reference: Search for "Break permission inheritance in SharePoint Online" on below article.
Customize permissions for a SharePoint list or library.
You can use Everyone or Everyone except External Users group in SharePoint to grant the permissions on your SharePoint site.
But make sure you are not granting them higher permissions than they should have on your site.
Ideally you should not give more than Read permission to this group or all users will be able to edit/modify lists items, documents, pages,...
Follow below steps:
At sub-site level, while breaking the permissions from Site collection select the options to create a new group and create groups for your sub-site as given below:
Then go to the home page of your sub-site.
Select Site permissions from Settings(Gear) icon.
Click on Change sharing settings under Sharing Settings.
Toggle Allow access ...
Users will need:
member of the farm administrators group
local admin on SharePoint server
Security Admin and DBCreator role on SQL
db_owner on all existing SharePoint databases
This blog post lays out additional details and recommendations, including a recommendation to grant these permissions to a group rather than individual users, so that you can add/...