Are there any sources about Meditations of Mahayana Schools?
For example, a great classical source on zazen would be Bendōwa by Dogen, which people jokingly pronounce bend over.
If you include Vajrayana under Mahayana umbrella, then there are bits on tantric meditation in Introduction to Tantra on Lama Thubten Yeshe, or Guru Yoga by Tsongkhapa, which is a manual. In the same subject of Samatha there is great treatise commentary in Alan Wallace's Stilling the Mind: Shamatha Teachings from Dudjom Lingpa's Vajra Essence from perspective of Dzogchen. There will be companion book by Alan Wallace Fathoming the Mind: Inquiry and Insight in Dudjom Lingpa's Vajra Essence that takes on another section (Vipassana) of Vajra Essence treaty.
Apart from that, many references directly pointing at texts can be found in this section of Buddhist meditation on Wikipedia.
The heart sutra is from mahyana schools of thought it's out there and one quite popular sutra .this trend of Buddhist thought teaches the message through meditation one upon inquiry about reailty , samsara etc is like a dream / dreams which appear and then disappear ...the meditation on Mahayana view is that all the mental problems ..joys knowledge etc are not real they will arise and subside .
The two I recommend:
Zhi-yi's Mohe Zhiguan
A meditation manual circa 594CE from the founder of Chinese Tiantai school, Zhi-yi (538–597CE) compiled from his lectures by his student.
Topics include: detailed instructions on samatha and vipassana; mindfulness in daily life, chanting, contemplation of the emptiness, meditation through prayer and sutra recitation, skillful means for easing one's mind and much more.
More information can be found on Wikipedia: Mohe Zhiguan
Dagpo Tashi Namgyal's Moonlight of Mahamudra
Fundamental work by a Kagyu & Sakya scholar Tashi Namgyal (1513–1587) providing exhaustive analysis of subtle points around samatha, vipassana, and mahamudra-style meditation.
More information can be found on Wikipedia: Dagpo Tashi Namgyal