Bike supports three file formats bike, opml, and txt as described in Using Documents. Each is what I would describe as an "open" file format, a format for structured data that is already supported by other applications.
The OPML format in particular is a common standard for outlines. If you want to share outlines with another app OPML is likely your best bet. Here is a list of OPML compatible apps on the opml.org website.
Bike Compatible Apps
This list welcome your feedback, additions, results, and notes. I have made this list by searching and reading product pages and release notes. Some things may be out of date or just untrue. Please let me know what needs fixing!
Bike (macOS) – Bike can read and write bike, opml, and txt directly to the file system as a supported file format. It can also read/write those formats to the clipboard.
Checkvist (Web) – Import and export OPML. Copy and paste outlines with Bike, may need formatting cleanup.
Drummer (Web + Electron) – Uses OPML as native format. The Electron version saves OPML directly to disk so you can work on the same file with Bike and Drummer without import/export. Both the web and Electron versions support import and export of OPML. Copy and paste outlines with Bike.
Dynalist (Web) – Import and export OPML. Copy and paste outlines with Bike, many need formatting cleanup.
OmniOutliner (macOS + iOS) – Import and export OPML. I think it can also use OPML as a standard document format (meaning no import export required). I think you can share an OPML document on iCloud and edit on iOS with OmniOutliner and on macOS with Bike. I have not tested this.
OutlineEdit (macOS) – Import and export OPML. Copy and paste outlines with Bike, may need formatting cleanup