Suppose you want to know what block or entity corresponds to a pixel on the screen. This can be done with pixel raycast.

All of this is client side.

There are two types, center pixel (crosshair) and arbitrary pixel.

This can be done with:

MinecraftClient client = MinecraftClient.getInstance();
HitResult hit = client.crosshairTarget;
 
switch(hit.getType()) {
    case Type.MISS:
        //nothing near enough
        break; 
    case Type.BLOCK:
        BlockHitResult blockHit = (BlockHitResult) hit;
        BlockPos blockPos = blockHit.getBlockPos();
        BlockState blockState = client.world.getBlockState(blockPos);
	Block block = blockState.getBlock();
        break; 
    case Type.ENTITY:
        EntityHitResult entityHit = (EntityHitResult) hit;
        Entity entity = entityHit.getEntity();
        break; 
}