Why does my cake lack air bubbles and looks like a molten mass?

This is a very vexing case, I douobt that anybody can tell from looking at your cake. So you will have to troubleshoot it yourself by first trying to bake a successful cake by following a traditional recipe and using best practices, and then, if that cake works well, start changing it back towards your preferred recipe one-by-one and seeing what makes the difference. For the test cake, you should bake a very standard recipe that has the best chances of rising well - I'd say that's a pound cake.

So, use following factors:

• 200 g eggs (4 whole eggs), 200 g white all purpose flour, 200 g sugar, 200 g cow-milk butter, 10 g baking powder (not baking soda!). Do not make any replacements here, and don't add other ingredients (flavors, etc.)
• make sure the ingredients are all room temperature, and have gotten there slowly. Just leave them overnight, no "Oh I forgot the butter in the fridge, I'll give it 15 seconds in the microwave".
• use the creaming method. Cream the butter and the sugar together until you see an obvious change in color and volume (can take 10+ minutes), then add the eggs and beat well, only then add the flour and baking powder.
• Use freshly bought baking powder
• sift the flour
• bake in a 175 C oven for as long as it takes to pass the skewer test.

If that works, as I said above, you will have to slowly change the ingredients back to your preferred recipe and see when the change happens.

If it isn't work even for that, there is some hidden factor that is very difficult to guess at. You will probably have to ask a good baker to bake together with you and either show you how they bake, or have them watch how you bake, and see if that person can spot the problem. You can also have them bake in your kitchen and with your oven, to see if they get the same trouble - but if you measured your oven's temperature, there isn't much that can be going wrong there.

Are you at a high altitude?

https://www.exploratorium.edu/cooking/icooks/article-3-03.html

Low air pressure has two main effects on baked goods: They will rise more easily, and lose moisture faster; liquids evaporate more quickly since water boils at lower temperatures at high altitude. As leavening occurs faster, gas bubbles tend to coalesce into large, irregular pockets in a batter or dough. The result? A coarse-textured cake. Alternatively, the pressure inside a rising batter can become so great, that cell walls stretch beyond their maximum and burst. Collapsing cell walls means the cake falls too.

It can be a challenge to get a cake to rise at altitude.

One of the reason could be that you do not whip egg whites or wait too long. You can try to add a spoon of lemon to the egg whites to avoid that. Another reason is that you need to use yeast. The cases depend a lot on the type of cake and the recipe followed (yeast is not always necessary).

Have you tried measuring the temperature in the oven? It looks to me as though your oven is too cool. Get an oven thermometer and give it a try. I had a fan oven that didn't fan properly, and my baking didn't rise properly. If I made the same recipe without fan (with 10% more heat) it worked fine.

Have you tried the same recipe in another oven?