There is a tendency amongst some players to bid in what one might call a “crawling style” and the hand shown is an example of this folly. Practitioners of this method bid just enough at each level and eventually, reluctantly, reach game and usually go down. This style is too kind to your opponents. They get room to discover any fits and how strong they are and then are given a free choice of bidding on or doubling you. When South produced the fatuous 1S overcall, West decided that her length compensated for the lack of high cards and introduced the heart suit. East can now see at least 12 tricks ( especially if partner has the expected KQD) and proceeds to check keycards to bid a grand slam. South belatedly tries to be a nuisance but the only reason the grand slam is not bid is that EW do not have any agreement about interference over Blackwood. One common agreement is DOPI ( Double shows 0, Pass shows 1). With that agreement, East could have bid 7H.
The play in 7H is simple, but the main interest is in how South should have bid.
South has a 12 count and expects much the same from East and so, on average, expects about 8 hcp from partner which are unlikely to be the correct cards for a slam. Also, it is clear South must worry about whether EW have a huge heart fit when South has little or no defence against a heart contract. Thus, it is clear that South’s first bid should be 4S. Now, the whole auction changes. West and North will pass. East, with such a strong hand, has a clear takeout double. With so few high cards, very few Wests will bid more than 5H and the slam will be missed due to the 4S overcall.
Lastly, note that 1C is the correct opening bid on the East hand. It is not strong enough to insist on game by opening 2C and is easy to express with a reverse ( 1C- 1S- 2H) or a jump in a new suit ( 1C- 1D- 2H).