It is estimated that the population of the world will peak around 2100 at 10.4 billion people. Even the population of China and India would stagnate & start drilling in the next three-four decades.
But many countries are already facing population decline. The high living standard coupled with the low birth rate is one of the main reasons for the fall of the population. But Western developed countries have managed to increase their population with immigration. But immigration to the West has devasted many countries (mainly in Eastern Europe) regarding their population increase. India has the largest diaspora in the world but its population is enormous and still increasing thus immigration has not yet affected the population of countries like India, the Middle East, Africa, etc. But Eastern Europe has stagnated population and the younger generation is moving to Western Europe. Countries like Bulgaria suffered heavily from outward immigration after joining the EU. Even Eastern European countries like Poland are suffering from outward immigration and thus had to allow immigration from further East (like Ukraine before the war). Even though many Russians have left the country in the last few decades but its population has not declined so much because Russia has got migrants from former Soviet Republics (mainly from central Asia). Ukraine has suffered an irreplaceable decline in population due to outward migration caused by the Russia-Ukraine war. Ukraine may not recover its population in the next few generations.
Image Source - visualcapitalist
Image Source - world population review
But there are some non-EU countries like Japan and Cuba whose population is decreasing. Japan has been forced to allow migration now as its population is fast aging and it can't keep its economy running like earlier. There has been competition for immigration in the EU. Countries like Portugal, Spain, and Italy have fallen behind Germany, the Netherlands, France, and Belgium in attracting skilled migrants and these countries can also witness population decline.