How come not much is known about Sinwar the Hamas leader Is he still in Gaza ?

Yes, he is still in Gaza.
Haniyeh was killed in Tehran on July 31, leaving Hamas with the task and difficulty of choosing a new leader.
Hamas's leadership is fragmented into three factions, each headed by Saleh al-Arouri in the West Bank, Yahya Sinwar in Gaza, and Khaled Meshaal overseas.
Haniyeh had the position of being the overall leader.
Israel carried out the assassination of Arouri in January, and more recently, Haniyeh.
After a series of careful considerations, the Palestinian shura council has chosen Sinwar as its new overall leader.
Sinwar was born on October 29, 1962, in the Khan Younis refugee camp.
In 1948, his parents were forcibly displaced from their residences in Majdal-Askalan due to Zionist ethnic cleansing, as Israeli settlers took control of the area and renamed it Ashkelon.
Due to the Zionist onslaught, Yahya's existence was filled with misery as he grew up as a displaced person under the Israeli military control of the Gaza Strip, which occurred in 1967, according to his father:

“Yahya’s life was full of agony due to the Zionist aggression. Since his childhood, he was determined to resist the occupation.”

Having excelled academically in school, he proceeded to pursue his studies at the Islamic University in Gaza. During his time there, he played a vital role in establishing the Islamic Bloc and assumed various leadership roles within the student council.
In 1982, Sinwar and several members of the student council embarked on a trip to Jenin to meet Palestinian women who were reportedly targeted by Israelis in a poisoning incident.
As a result of this visit, he was detained and sent to administrative detention, which means being kept without charge or trial, for a period of six months. He was accused of engaging in subversive Islamist activities.
While in detention, Sinwar formed relationships with fellow activists, including Saleh Shehade, who later assumed leadership of Hamas' armed faction until his assassination in 2002.
Sinwar was tasked with establishing a comprehensive security network, referred to as Majd.
The Majd functioned covertly while the Mujamma Islamiyya, an organization linked with the Muslim Brotherhood that came before Hamas, maintained a non-combatant status until the formation of Hamas in late 1987.
In 1988, Sinwar was apprehended and purportedly subjected to severe physical abuse for a duration of 6 weeks subsequent to the unearthing of armed factions affiliated with the Majd.
In 1989, Hamas conducted its inaugural substantial act of armed aggression, resulting in the demise of two Israeli soldiers. Sinwar was convicted on allegations of orchestrating the attack and given a prison sentence of 426 years.
As the most prominent Hamas leader released in a prisoner exchange agreement in 2011, Sinwar went back to Gaza and was later elected as the leader of Hamas in the Strip, succeeding Ismail Haniyeh.
In 2017, Hamas underwent a process of rebranding and updating its statute, signaling its willingness to consider and embrace a Two-State Solution as part of its stance as the Islamic Resistance Movement.
In that particular year, Sinwar played a significant part in endeavoring to mend relations between the Palestinian Authority (PA), which is headed by the Fatah Party, and Hamas. However, his efforts were unsuccessful.
In 2018, Hamas, led by Yahya Sinwar, embraced a policy of non-violent resistance in order to facilitate diplomatic negotiations aimed at resolving the Gaza siege.
The Hamas leadership officially supported the large-scale nonviolent protest action, referred to as the "Great March of Return," that commenced on March 30, 2018.
Nevertheless, in response to the US's unilateral recognition of Jerusalem as Israel's capital and the Israeli troops killing of numerous unarmed protesters, Hamas altered their policy once more.
In May 2021, Hamas initiated the Saif al-Quds campaign in retaliation to Israeli aggression on Palestinians in Jerusalem and the West Bank, receiving backing from several armed factions within the Gaza Strip.
Subsequently, Yahya Sinwar's orations and public appearances have propelled him to become a highly esteemed and influential figure in the Arab World.
I perceive the election of Sinwar as carrying profound significance and symbolism:
  1. This indicates that Hamas, including all its divisions, remains cohesive and undivided.
  2. Hamas perceives the Resistance in Gaza as being resilient, cohesive, well-structured, and competent in leading an ongoing struggle against the Israeli occupation.
  3. The claim that there is a struggle between Hamas' 'moderates' and 'hardliners', as reported by several media outlets including major US media, is false.
  4. Hamas remains committed to supporting the Sinwar strategy of Resistance, even after more than 300 days of war.
  5. Hamas has become much more powerful and cohesive following the targeted killing of its leader, Haniya.
  6. Despite the ongoing Israeli war and slaughter in Gaza, it is important to recognize that Hamas is a structured movement with established institutions. Decisions inside Hamas are made through a democratic process, which continues to be operational despite the ongoing genocide in Gaza.