Is it considered zina (illicit sexual intercourse) that necessitates the ḥadd punishment if part of a man's penis or his belt enters the vagina, knowing that the belt was unfastened? And does penetration through a thick barrier necessitate the ḥadd?

If a Muslim falls into the act of zina (fornication/adultery) — a reality beyond doubt — then repentance is obligatory upon them. However, they are not required to confess; rather, it is sunnah (definition) for them not to confess and to conceal themselves. One who repents from a sin is like one who has no sin, and Allah forgives them and transforms their evil deeds into good deeds. The fornicator/adulterer is not obligated to confess in order to have the hadd punishment applied, and scholars have mentioned that it is recommended for one who has confessed to retract their confession. The Sharia (definition) is keen on concealment, so whoever falls into such a sin should conceal themselves with Allah's concealment, improve their future conduct, increase in performing good deeds, attend gatherings of remembrance (dhikr (definition)), accompany righteous women, and avoid everything that leads to temptation. Suicide is not a solution; rather, it is a disease by which the one who commits it transitions from one misery to a greater misery.