If you're talking about synth pedals, then there are two main types:
Waveshapers, which take your bass's signal and change the waveform in some way, usually by distortion or filtering, to make it more 'synthy'. They may also use an octave effect to double your bass's pitch an octave or more above/below; and
Pitch trackers, which use oscillators like those on 'real' synthesisers, but change the pitch of the oscillators based on your bass's signal.
After that, both types process the audio with filters (like an envelope filter, to change the harmonic content of the signal) and amplifiers (to change the level of the signal), both possibly controlled by envelope generators (to control changes to filtering and amplitude over time).
They may also have low frequency oscillators (which can introduce slow changes to pitch/filter/amplitude over time, like vibrato or tremolo).
The main difference between the two types is in the sound generation - the waveshapers have excellent response to your playing, since they just modify your bass's signal, but you might find that the sound is too fuzzy or not synthy enough for your tastes.
The pitch tracking types sound more like 'real' synthesisers, because they use they same analog or digital oscillators, but tracking the pitch of the note you are playing on your bass is not easy, and these types often have tracking issues, which may result in glitches, wrong notes, delays before playing notes etc.
While there are many all-in-one synth pedals (Akai Deep Impact, EHX Bass Micro Synth, MarkBass Super Synth, Boss SYB5, Ibanez SB7, Behringer BSY600 to name a few), many players experiment with different pedals, along the lines of the effects you mentioned, to create synth sounds. A typical chain of pedals might be:
Fuzz - Octave - Envelope Filter - Chorus/Phaser/Flanger - Reverb
There are a million threads on bass synth effects here - try this or just search for 'synth': Whats the best synth effects pedal for bass?