How to track stationary bike activity with the Apple Health App on an iPhone SE?
There is a chart I found on the American Exercise Council...step conversion table...minutes of lots of activities at various levels of effort convert to so many steps per minute...such as stationary bike...I bike for 40 min at a time..( 2 programs that automatically change resistance)...this converts to 19 steps a minute of s bike activity....I initially Googled something like Step/Bike Conversion)

So, if you like that sort of thing (manually calculating) there are ways to do that without the Apple Watch or third party apps.