The bot tells me G12 was my losing move. Instead, |13.n12 14.n20 15.s17 16.s20 17.p20 18.q19 19.l18 20.m8 21.i7 22.j7 23.k8 24.l8 25.j5 with advantage to white.

Why not |25.f19?

Then 26.f21 27.g21 28.h22 29.i22 30.j23 31.k23 or 26.g20 27.j21 28.i21 29.l20 30.k22 31.l22 - am I missing something?

Probably, because after 25.f19 is followed by 26.g20 27.j21 28.i21 29.l20 and then 30.k22

After 19.m18, I replied with 20.o15 on the spot, without considering the situation. But after 21.p16, I found a lot of traps that David had set up. My 22.m22 was a desperation move, because I did not find anything better. In this respect, I wonder if 23.o18 is appropriate, because I replied with 24.o21 and I built up a more solid defense.

My previous comment is wrong, because after 30.k22 31.l22 and I cannot stop it. Alan is right.

|25.f19 would have been a better practical decision for me, but the bot finds an amazing resource with 26.i6

The point is, white can connect either the left or the right to the bottom, and black can choose which. In order to win, white must be able to force both the left and the right groups to reach the top or connect these groups. For example 27.m7 28.m6 29.l5 30.n8 31.n9 and now 32.g20 33.j21 34.i21 35.l20 36.o11 37.t8 does not quite work, but 32.l4 33.p10 34.q11 35.k3 36.g20 does.

Compare this to the |13.n12 line in my first post, which effectively connects left to right, and the tactics to reach both the top and the bottom become simpler.

It may be simpler for me to say, after |25.f19 26.i6 27.m7 28.m6 29.l5 30.n8 31.n9 32.g20 33.j21 34.i21 35.l20 black should avoid 36.o11 37.t8 but instead play 36.l4

Another variation is 27.n7 28.n9 29.p6 30.l8 31.t9 32.l20 33.g21 34.m11 or 29.m9 30.g20 31.j21 32.i21 33.l20 34.r8