Visual fade in/out key command

This would not work the same way as a transition and you wouldn't be able to save any presets you might want to make defaults. I made transitions that would fade to/from transparency, but I had to make them either with a popup option to be transparent incoming or transparent outgoing — or two separate transitions. There's no way (that I currently know of) that the motion template can tell which side you mean to be transparent. The effect is the best way.

You can set up any of your presets as a default effect by right clicking on the preset and Make Default Video Effect. I think the keyboard shortcut is ⌘/ (command-forward slash). It's easy to change what you want as a default effect if you need to adapt to some other aspect of your editing.

Instead of transitions, try this:

https://fcpxtemplates.com/sdm_downloads/fade-in-fade-out/

It's a basic "behavior" from Motion that is "missing" from Final Cut. Install as an effect.

You can customize the settings and Copy the clip you make settings in then Command-Shift-V (Paste Attributes) to all other selected clips. You can also save your customized settings as Effect Presets. Later on, you can simply select all your clips you want to fade and double click on either the effect or one of your presets to apply them all at once to all selected clips.

HTH

Sorry, I left out a confirmation key press.

"Select all the clips you want to fade up and down and type Ctrl-D, then -1, then press Enter. This will shorten their duration by by 1 frame. Then, with them still selected, type Cmd-T to add fades to the start and end."

If the clips are butted up against each other, adding a dissolve with Cmd-T will dissolve between them. Leave a couple second gap between the clips and apply a dissolve to the end of a clip and another to the beginning of the next. You can grab the top left or right of the dissolve to trim the underlying clip. You need to be zoomed in enough to see the "handles" which are two, vertical lines on either side.

Adjust transitions in the timeline in Final Cut Pro for Mac - Apple Support

Try this for a much faster way than manually editing opacity. Select all the clips you want to fade up and down and type Ctrl-D, then -1. This will shorten their duration by by 1 frame. Then, with them still selected, type Cmd-T to add fades to the start and end.

If you're adding Cross Dissolves (fade in/out) to overlaying clips, then if you right click on the transitions, you can choose to turn off Transition To/From Black:

This will result in a fade to transparent. I do not believe there is a way to make fade to transparent the default option.

I’m not clear what you want to appear between the clips. Do you just want a fade out to the clip below and then a fade in to the next connected clip? Separate the two clips by a frame so you can select the end point and add the cross dissolve. It will do a fade out to the clip below. Select the start edit point on the next clip and add add the cross dissolve.

Cmd-T is set for any transition you assign as the default. If you make fade to color the default, Cmd-T will add a fade to black between two clips.

The clips are separated by one frame. In the image you posted earlier the clips are too close together to see a one frame separation. You have to zoom into the timeline a lot to see a gap of a single frame.