Children explain addition is starting with more than 1 group, and joining the groups together, and subtraction is identifying which number we could make a group of that would give us enough to take away the other number, then starting with 1 group, and taking a group away, and investigate adding and subtracting a single-digit number bridging any decade, using the place value concepts they have been investigating.
As children are investigating adding and subtracting bridging any decade, they continue to investigate the place value concepts of friends of any decade (Place Value 10), and place value of, and partitioning two-digit numbers (Place Value 11).