Reading the Quran is not the same as studying a novel or even a scientific literature. Understanding a single verse requires a huge effort if we want to analyze it in the right context. And even so, there is a lot uncertainty, as there are still veils of ignorance covering many ayas. We haven’t gotten there yet with our knowledge to understand a lot from the meaning. Maybe the centuries to come will also pave the way for understanding those that we are unable to comprehend today while we provide forced explanations.There is no single, fixed understanding of the doctrines of the Quran (Aqida) and of the Order of the Divine Legitimacy (Sharia). This should be imagined as an ocean into which we dip a spoon at every age and take out a few drops of water. This amount of water means the comprehension of the doctrines and ordinances of a given age. The understanding of that few drops of water from legal points of view in a given age is called Fiqh. So, no one can understand Sharia in its full reality. Only the interpretation of Sharia from the given age, i.e. Fiqh appears, which is only a fragment of the whole of Sharia in relation to that age and society. As all sciences evolve, so does the understanding of the Quran. Based on this, schools and tendencies of doctrines and legal interpretations are created, which is not only an opportunity, but statutory. These schools need to provide the appropriate interpretations and answers to the current questions of the particular age. So, we don’t have to look for answers to the questions of the past! There is a hadith about the coming in every hundred years of someone (Mujaddid) who cleanses religion from vile accumulations, false innovations of faith (bida’a) and renews the understanding:

Narrated Abu Hurayrah:
The Prophet (peace be upon him) said: Allah will raise for this community at the end of every hundred years the one who will renovate its religion for it.
Abu Dawud said: 'Abd al-Rahman bin Shuriah al-Iskandarani has also transmitted this tradition, but he did not exceed Shrahil.
Geade: Sahih (Al-Albani)
Reference: Sunan Abi Dawud 4291

The hadith by no means speak about Awliya' Allah, but about scholars who do some kind of Ijtihad to renew the religious understanding for the people either by going back to the roots or with a new understanding/interpretation or showing off what went wrong during the last period or by pointing at things that people have committed, missed or not taken into account... so "renew" in the context might have a lot of meanings and the hadith by may be understood that each 100 years there will be scholars (not necessarily a single scholar) who may renew the understanding of religion.
More simply speaking it means Allah will send this Ummah scholars or people of knowledge and guidance who will guide people back to the right path each time a majority out of them might deviate.

Some of the scholars who have been considered as renewers:

1. 'Omar ibn 'Abdal'aziz the caliph at the end of the first century.
2. Imam a-Shafi'i (Second century)
3. ibn Sarij (Shafi'i) and al-'Asha'ari (according to imam as-Suyuti's poem -see here in Arabic- on the mujadid's "renewers") some said at-Tahawi (Hanafi) and imam an-Nasa-i. Wikipedia also quotes the Shi'a hadith scholar al-Kulayni (Third century)
4. Al-Baqilani (Maliki), abu Sahl as-Su'luki (Shafi'i) and al-Isfiryani (Shafi'i) (according as-Suyuti's poem), al-Qadir Billah (the caliph), al-Khawarizmi (Hanafi), al-Hakim from Nishapur and ibn Hazm were also mentioned. (Fourth century)
5. Al-Ghazaly (Fifth century)
6. Fakhr ad-Dyn ar-Razi, ar-Rifa'i (both Shafi'i) (Sixth century)
7. Ibn Daqiq al-'Iyd (according as-Suyuti's poem) and ibn Taymiyah Wikipedia also quoted ibn 'Arabi and the sufi al-Chichti (Seventh century)
8. Al-Balqini, Zayn ad-Dyn al-'Iraqi (both shafi'is and according to as-Syuti's poem), Wikipedia quotes ibn Hajar al-'Asqalani (Eighth century)
9. Jalalul ad-Dyn as-Suyuti. According the Wikipedia Article (Ninth century)
10. Ar-Ramly. (Tenth century)
11. Sirhindi, Shah Walliullah ad-Dahlawi and 'Abdullah ibn 'Alawi al-Haddad. (Eleventh century) ...

Two more detailed list appears in Arabic Wikipedia: see here and here which is the content of an Encyclopaedia on the renewers.bAlso English Wikipedia: https://en.wikipedia.org/wiki/Mujaddid Some of my references the fatwas: on islamqa #153535 and islamweb #74655 (in Arabic language).

For more details, see Chapter 1.5, “Schools of Islamic Thoughts,” and Section 2.3.7. “Main Schools of Jurisprudence”.

There is a general consensus among scholars that it signifies over a period of every 100 years Muslims will stray from the straight path. Many innovations (bidah) will enter the religion. Hence, Allah sent someone who was brought up with the true understanding of the Quran and Sunnah. This person will be a beacon for the people of the time.You will have to elaborate on the term “Wali ul Allah” because unfortunately many people have abused the term for their personal benefits. A true Wali ul Allah is someone who has submitted himself to God while remaining in this world and handling both deen & dunya. He/she is not after worldly possessions nor fame but works only for what is required to sustain himself and his family. Everything he/she does is for the pleasure of Allah and is based on the Quran & Sunnah.
There were a multitude of such people in the days gone and will be very few in the present and coming days.

In the light of the above I ask: what is the achievement of the many Islamic conferences, discussions that are held at a huge cost by countries that are said to be Islamic, competing with each other?