Why do my colours completely fade when I merge layers or export to jpeg?

It is due to the way PS flattens an image; by default it (sometimes?) preserves the layer masks; and the underlying layer's mask will mask the layer above. If you attempt to merge/flatten only two layers PS will pop up a warning about this; and it gives you an option of applying the mask of the underlying layer before merging. You do not get the option when you export, flatten a group of layers, or flatten the entire layer stack.

The behavior also seems to be dependent on what is being flattened/merged e.g. adjustment/vector layers vs image layers (or if an image layer is included). I can't really say for certain exactly when it will/won't happen. In your case, it appears that the layer "1"s mask is masking some/all of the edits above it...

The way I have found to get around it is to create a new top layer and then merge visible into it (shift-opt-cmd-E). This copies the current view as a new image on top of all other layers; then turn off all other layers below before exporting/flattening (alt/option click on the top layer's view (eye) button). Your result should then be what you were expecting.

The sample image you've posted uses an Apple Display Device Profile. Whatever else is going on, you probably are experiencing color space issues. If your work is intended for online presentation, change the working and output color spaces to sRGB.

Here is what exiftool reports:

[ICC_Profile]   Profile CMM Type                : Apple Computer Inc.
[ICC_Profile]   Profile Version                 : 4.3.0
[ICC_Profile]   Profile Class                   : Display Device Profile
[ICC_Profile]   Color Space Data                : RGB
[ICC_Profile]   Profile Connection Space        : XYZ
[ICC_Profile]   Profile Date Time               : 2020:11:20 02:34:52
[ICC_Profile]   Profile File Signature          : acsp
[ICC_Profile]   Primary Platform                : Apple Computer Inc.
[ICC_Profile]   CMM Flags                       : Not Embedded, Independent
[ICC_Profile]   Device Manufacturer             : Apple Computer Inc.
[ICC_Profile]   Device Model                    : 
[ICC_Profile]   Device Attributes               : Reflective, Glossy, Positive, Color
[ICC_Profile]   Rendering Intent                : Perceptual
[ICC_Profile]   Connection Space Illuminant     : 0.9642 1 0.82491
[ICC_Profile]   Profile Creator                 : Apple Computer Inc.
[ICC_Profile]   Profile ID                      : d68c301538f7f7532ee768b3d8951f27
[ICC_Profile]   Profile Description             : Color LCD_20-11-2019 try 1.icc
[ICC_Profile]   Profile Copyright               : Copyright Apple Inc., 2020
[ICC_Profile]   Media White Point               : 0.9642 0.99998 0.8252
[ICC_Profile]   Red Matrix Column               : 0.51721 0.24384 0.00215
[ICC_Profile]   Green Matrix Column             : 0.29042 0.69925 0.03989
[ICC_Profile]   Blue Matrix Column              : 0.15656 0.0569 0.78316
[ICC_Profile]   Red Tone Reproduction Curve     : (Binary data 16 bytes, use -b option to extract)
[ICC_Profile]   Blue Tone Reproduction Curve    : (Binary data 16 bytes, use -b option to extract)
[ICC_Profile]   Green Tone Reproduction Curve   : (Binary data 16 bytes, use -b option to extract)
[ICC_Profile]   Video Card Gamma                : (Binary data 1583 bytes, use -b option to extract)
[ICC_Profile]   Chromatic Adaptation            : 1.04782 0.0229 -0.05006 0.02956 0.99045 -0.01703 -0.0092 0.01503 0.7524
[ICC_Profile]   Luminance                       : 114.04559 119.99817 130.61426
[ICC_Profile]   Make And Model                  : (Binary data 40 bytes, use -b option to extract)