If a particular ssri doesnt help depression/anxiety symptoms, how does switching to a different ssri help since they both block reputable of serotonin? are all ssris chemically different in some way, so while one may not work, another one will?

If your SSRI has been taken for a few weeks & hasn't been effective, it's worthwhile switching to a different SSRI as efficacy can be highly individual with these meds. However,the transition must be done carefully & must be supervised by your prescribing physician in order to minimize withdrawal problems, side effects or aggravating your original symptoms.