How does recv () understand that all data is received in TCP cases?

As far as I understand, data over TCP is transmitted as a continuous stream, until the connection is broken. If you look at the structure of the TCP segment, there is not even information about the length of the data (unlike UDP, for example). Thus, if we read something from a TCP socket to the buffer, the read will occur until the connection is closed, or the buffer overflows.

However, if you look at the actual code, this is not the case-recv () on the server reads exactly as many bytes as are sent from the client using send()

How does recv () understand that all the data is received and the control needs to be returned to the calling code?

Complete and minimal example on bare sockets:

Server:

#include <iostream>

#include <sys/socket.h>
#include <unistd.h>
#include <netinet/in.h>
#include <memory>
#include <arpa/inet.h>

const int BUFFER_SIZE = 1024;
const int PORT = 12345;

int main() {
    //create server socket
    int socketFd = ::socket(AF_INET, SOCK_STREAM, 0);
    if (socketFd < 0) {
        return -1;
    }
    int opt_val = 1;
    setsockopt(socketFd, SOL_SOCKET, SO_REUSEADDR, &opt_val, sizeof opt_val);


    //bind to address
    sockaddr_in socketAddress;
    socketAddress.sin_family = AF_INET;
    socketAddress.sin_port = htons(PORT);
    socketAddress.sin_addr.s_addr = htons(INADDR_ANY);
    int rc = ::bind(socketFd,
                    reinterpret_cast<sockaddr*>(&socketAddress),
                    sizeof(socketAddress));
    if (rc < 0) {
        return -2;
    }


    //listen
    rc = ::listen(socketFd, SOMAXCONN);
    if (rc < 0) {
        return -3;
    }

    //accept new connection
    sockaddr_in socketAdress;
    unsigned int sizeOfSocketAdress = sizeof(socketAdress);
    int clientSocket = ::accept(socketFd, (struct sockaddr *)&socketAdress, &sizeOfSocketAdress);
    if (clientSocket < 0) {
        return -4;
    }

    //receive
    char buffer[BUFFER_SIZE];
    int receivedBytes = ::recv(clientSocket, buffer, BUFFER_SIZE, MSG_NOSIGNAL);

    std::cout << "Received " << receivedBytes << " bytes : "  << buffer << std::endl; // Прочитано 5 байт "hello", хотя буфер не заполнен и соединение не прервано
    return 0;
}

Client:

#include <iostream>

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

int main() {
    struct sockaddr_in sa;
    int res;
    int socketFd;

    socketFd = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP);
    if (socketFd == -1) {
      perror("cannot create socket");
      exit(EXIT_FAILURE);
    }

    memset(&sa, 0, sizeof sa);

    sa.sin_family = AF_INET;
    sa.sin_port = htons(12345);
    res = inet_pton(AF_INET, "127.0.0.1", &sa.sin_addr);

    if (connect(socketFd, (struct sockaddr *)&sa, sizeof sa) == -1) {
      perror("connect failed");
      close(socketFd);
      exit(EXIT_FAILURE);
    }

    auto buf = "hello";
    auto len = 5;

    int sentBytes = ::send(socketFd, buf, len, 0);

    std::cout << "sent " << sentBytes << "bytes: " << buf << std::endl;

    std::string tmp;
    std::getline(std::cin, tmp); //приостановка выполнения, соединение все еще не закрыто
    return EXIT_SUCCESS;
}