Villager not breeding still

The issue seems to be you have too many doors without sufficient space "in front of the door/inside the house" and "behind the door/outside the house" for each of the doors in the underground area. None of these are really considered a house for villager breeding purposes.

As @fabian indicates, the mechanics of villager breeding is more complicated than placing a sufficient number of doors. What you need to accomplish is to trick minecraft into thinking you have created a village. Only then will the villagers start breeding.

Here is a link that may help you: https://minecraft.gamepedia.com/Villager#Breeding. There are also a number of videos online that will explain how to create a working villager breeder.

(FYI the mechanics of villager breeding changed with different versions of Minecraft, and are likely dependent on which platform you are running on, so if you are reading this in the future, your mileage may vary).

Out of personal experience, I'd say the following should help in 2021, as of version 1.16.5:

• Build houses (at least 3x3 on the inside).
  • Put beds in there.
  • Building a large house with 3x3 rooms also works.
• Give bread to your villagers, just throw it on the floor next to them, they will pick it up (you want mob griefing turned on for that).
• Have a trade with them. You can farm a lot of emerald selling carrots to most farmers. You can spend it on silly things, villagers tend to sell much better items when they level up.

And that's pretty much it. This should be sufficient for normal gameplay unless you want to build some crazy farm of some sort.