More about laws concerning work on the Sabbath in the Old Testament
The Pharisees felt that the disciples ,in plucking grain and eating it, were working on the Sabbath. So what were the laws concerning the Sabbath. Well we read in Exodus: 'For six days shall work be done, but the seventh day is a sabbath of solemn rest, holy to the Lord.' (Exodus Chapter 31.15)
But by the time of Jesus things had been tightened down so that specific things were deemed work, and strict Jews would avoid them.
For anyone wanting to look up a good, but long, article about this, look at: https://evidenceforchristianity.org/what-is-the-definition-of-work-in-the-old-testament-and-how-were-people-to-know-what-constituted-or-did-not-constitute-workr/