Answer
When we say that heaven and hell are on earth itself, what we are really saying is that one can experience heavenly happiness on earth and one can also experience hellish suffering on earth. Both of these come to us in accordance with our own karma and our own nature. However, this does not mean that there is no heaven or hell after death. After death, the soul either takes rebirth — as a human being, or as an animal, bird, or insect. If one has committed excessive sinful acts, the soul goes to hell and endures torment there for a very long time. If one has performed great meritorious deeds, the soul goes to heaven and enjoys heavenly pleasures there for a long time.
Heaven exists, and above heaven there are several other lokas (realms) where the soul receives bliss of the highest order after death.
Regarding what happens to the soul after death, there is a detailed video available on our YouTube channel that we made explaining this at length. Watching it will give you better knowledge on this subject. But let me mention here as well that after death, before taking rebirth or before going to heaven, hell, or other destinations, the soul exists as a preta (disembodied spirit). Some souls remain as pretas for a short time, while others remain as pretas for hundreds of years. A preta has no gross body through which it can experience pleasures, but it possesses a subtle body composed of the subtle essences (tanmatras) of the five elements — earth, water, fire, space, and air — along with mind, intellect, and ego, in which all its memories and samskaras (accumulated impressions) are stored. Carrying these memories and samskaras, the soul enters another body when the time comes, in accordance with its karma. As long as it remains in the preta yoni (state of being a disembodied spirit), it suffers greatly, because it retains desires for sensory enjoyment but, lacking a physical body, it cannot fulfill them. Therefore, remaining in the preta yoni is not considered a good thing. For this reason, in Hindu Dharma, a thirteen-day ritual is performed after death so that the preta may be given a makeshift, small body through which it can obtain some minimal sustenance — for instance, so that its thirst may be quenched when water is offered, and its hunger may be satisfied when food is offered, and so on.
However, this ritual does not grant liberation from the preta yoni — it only provides some relief from hunger and thirst and gives the soul some support for its onward journey. To grant liberation from the preta yoni, the recitation of Bhagavat Katha or the Bhagavad Gita is prescribed. Through this, the soul moves beyond the preta yoni and either takes birth or reaches its destined heaven or hell.
Some meritorious souls depart quickly for heaven, while others remain as pretas.
Once a person gains some clarity about life after death, he begins to think about preparing for it — about how to ensure that he does not suffer after death. Therefore, I urge you to watch the video I mentioned, which is available on our channel.
Those who are virtuous, who perform good deeds, and who are also elevated devotees of the Lord — such souls do not have to pass through the preta state. Upon leaving the body, they are taken directly to Vaikuntha. According to the scriptures, they are not even required to appear before Yamaraja (the Lord of Death).
The aforementioned gati — the attainment of Vaikuntha — is the destiny of devotees of Narayana.
Those who are Jnana Yogis have no "gati" (movement to any realm) after death. Upon leaving the body, they merge into the formless Brahman (Nirakara Brahman).
If you study the Bhagavad Gita, you will find many verses that present a general picture of the state of the soul after death.
If you come to know precisely what states your soul may pass through after death, you can begin taking action right now to ensure a good afterlife. By doing so, you can joyfully attain the excellent Svargaloka, Brahmaloka, or the supreme Vaikuntha after death. Alternatively, your rebirth will be far better — you will not take birth in animal or insect species, but in the human species, and in a good family, and your life will be better.
Bear in mind that many Hindu scriptures and the priestly class prescribe elaborate and expensive rituals for attaining a good afterlife — for the benefit of the priestly class itself. Do not fall into that trap. Your destiny is determined ninety-five percent by your own good deeds. Charity and service to others are foremost among them. Courteous behavior toward all and gentle speech are foremost among them. Chanting the Lord's name, kirtan, and daily reading of the Gita, the Bhagavatam, and the Ramakatha — these are the essentials.