It uses all the cores, or rather: you can instruct Logic to work in either of these ways:
I just made a quick test with Modern Scoring Strings, having five Kontakt instances open at the same time, with unique instruments – all with four dynamic legato layers and automation of dynamics and vibrato. This was on an 2020 i7 3,8 GHz iMac, and it played all these tracks without using more than between 50% and 75% of one single core.
I may be wrong about this, but I'll give it a go: I think it's only if one puts so much stuff on one single track that one core cannot handle it that Logic can't switch to using multiple cores for one single track. But since this test shows that one core can handle 20 dynamic layers of legato/vibrato and a total of 8 mic positions activated, the situation isn't really bad. There are iPads out there with better performance than this iMac (which has the best single core performance of all Intel Macs), but of course YMMW.
When playing live (four out of the 5 instruments from the playback tracks, copied to a separate track), the meters looks like this:
If I try with five
MSS instruments on one track instead of four (in addition to the 25 playback instruments spread over 5 tracks), the meter in the core used for live playing hits the roof. All this is with a 256 buffer in Logic.
The performance in the M2 and M1 Macs is of course better than this.