How is samsara nirvana?

Prajñāpāramitā Hṛdaya sūtra: « There is no suffering, no cause of suffering, no end to suffering, no path to follow »

Where beauty is, then there is ugliness;
where right is, also there is wrong.
Knowledge and ignorance are interdependent;
delusion and enlightenment condition each other.
Since olden times it has been so.
How could it be otherwise now?
Wanting to get rid of one and grab the other
is merely realizing a scene of stupidity.
Even if you speak of the wonder of it all,
how do you deal with each thing changing?

-Ryokan

As @Nemo says this is the highest truth that is grasped fully only by self-realization. It would be ineffectual to explain it. This being the case, why did the enlightened ones speak about it?

For two reasons.

Firstly, these quotes are mysterious statements designed to awake in one the deep introspection that leads one to awakening.

Secondly they act as sign posts to dispel doubt when such contrary views are realised for oneself.

These were expressed more frequently in Japanese Zen in times when the monastic order had become excessively institutionalised, and monks started to expect the laity to honour, respect and feed them as if they laity were a lower order of being. At this time the enlightened ones recalled these views to show the right way. It is not meant as a a ridicule or insult of tradition, but a gentle nudge to not forget the ideals and get caught in value judgements and dualistic discursive convention.

Outwardly they pretend to be superior, praying on the gullibility of old peasant women and congratulating themselves on their cleverness. Alas! Will they ever come to their senses? A monk would sooner walk among a pride of mother tigers than tread the path of fame and fortune.

The Vimalakirti sutra is important and well known in the Mahayana tradition, and is famous for expressing this higher ideal.

The story in the sutra is Vimalakirti, the lay Buddhist known to be a Bodhisattva falls ill, and he uses it as a convenient excuse to teach everyone who visits him. Even in his period of convalescence he spreads the truth of the Bodhisattva way.

Manjushri rides on the lion
Samanthabhadra mounts the elephant
Ghosa enters nirvana on his jeweled seat
And Vimalakirti lies ill on his bed

Here Ryokan compares the sick bed of Vimalakirti to the mounts of fabled Buddhist teachers. In his sickness Vimalakirti embodies the Bodhisattva's grasp of non-duality, using it to teach many.

Samsara and nirvana are subjective points of view. Each is experienced by you by means of your mind. Whether something is pleasant or unpleasaant is a subjective judgement created by the 5 skandas, which are your own personal, unique, and subjective sensations, perceptions, mental formations, and consciousness. The objective world "out there" is devoid of your subjective skandas.

Samsara is Nirvana in at least two senses:

Nirvana is achieved here, in this world (="samsara"). We don't fly to another planet or go to other dimension when we achieve Nirvana; our perception changes, and so does our subjective reality.

Samsara is Nirvana because we should stop trying to escape. Trying to escape "this" is what makes it Samsara. The idea of otherworldly Nirvana is exactly the kind of craving that leads to suffering. Samsara is Nirvana means one has stopped craving. That is called cessation.

How I understand it, "what is" seen as divided is samsara (with self, non-self etc) and seen as undivided is nirvana (with no independent self etc). So samsara and nirvana are the same stuff viewed differently, in other words samsara is the superficial view and nirvana is the deep view of the same stuff.

There is this beautiful story, once Buddha preached to some of his disciples when they ask about Nirvana.

They asked from Buddha, "Blessed one, please tell us what Nirvana looks like, feels like. So we can try to attain it." Buddha said nothing. They asked several times and Buddha continued his silence. When these disciples went out, he preaches others below story.

When I am (Buddha) going on a journey, I saw this house full of flames. It is burning so badly with everything in it. And I saw some people still in there. I went to this burning house, open the door and said them, "This house is burning. Please come out". Instead of coming out, these people asked me, How is outside, is it raining? will there be a place for us if we leave this house etc". So I didn't tell them anything. Just came out.

The The Jewel Ornament Of Liberation (which I have only begun to read) has some paragraphs near its beginning which juxtapose samsara with nirvana:

In general, all phenomena are included in the two categories of samsara and nirvana. That which is called samsara is empty by nature, a confused projection. Its defining characteristic is that it manifests as suffering. That which is called nirvana is also empty by nature, but all the confused projections are exhausted and dissipated. Its defining characteristic is freedom from all suffering.

And:

III. HEARER FAMILY

The family of Hearers consists of those who fear samsara and yearn to achieve nirvana, but who have little compassion. It has been said:

One who is afraid upon seeing the suffering of samsara
And yearns to achieve nirvana
But has little interest in benefitting sentient beings—
These three are the marks of the Hearer family

IV. SOLITARY REALIZER FAMILY

The Solitary Realizer family includes those who possess the above three attributes and in addition are arrogant, keep their masters’ identities secret, and prefer to stay in solitary places. It has been said:

Fear at the thought of samsara, yearning for nirvana,
Little compassion, arrogance,
Secretive about their teachers, and enjoying solitude—
A wise one should understand that these are the marks of the Solitary Realizer family

So these two families, the Hearers and the Solitary Realizers, engage in their respective vehicles and even though they achieve the results of their practices, these results are not the final nirvana. How do they abide when they achieve their fruits? They maintain unafflicted states of meditative concentration, but those states are based on the psychic imprint of ignorance. Since their meditative concentrations are unafflicted, they believe that they have achieved nirvana and remain that way.

If their states are not the final nirvana, then one might argue that the Buddha should not have taught these two paths. Is there a reason the Buddha should teach such paths? Yes. For example, suppose great merchants from this Jambudvipa are traveling the ocean searching for jewels. After many months at sea, in some desolate place, they become completely tired and exhausted and think, “There is no way to get the jewels now”. When they feel discouraged and prepare to turn back, the merchant captain manifests a huge island through his miracle power and lets all his followers rest there. After a few days, when they are fully rested and relaxed, the captain says, “We have not achieved our goal. Now we should go farther to get our jewels.”

Similarly, sentient beings without courage are frightened when they hear about the Buddha’s wisdom. They believe attaining Buddhahood is a great hardship, and think, “I have no ability to do this.” There are other people who are not interested in entering the path, or who enter the path but turn back. To counter these problems, Buddha presented these two paths, and allows them to rest in these states.

As said in the White Lotus of Sublime Dharma Sutra:

Likewise, all the Hearers
Think that they achieved nirvana,
But they have not achieved the final nirvana
Revealed by the Buddha
They are only resting