I live in NY and work 100% remotely for a company based in CA (will never have to travel to CA). My job is currently withholding NYS tax only. Should I be paying quarterly estimated taxes to CA?
You do not live or work in the state of CA. It is very common to be employed by an employer whose HQ is in another state. You pay state tax for the state you live and work in---which is New York. You do not need to file a CA tax return just because your employer is based in CA.
If you're a W-2 employee and you will never physically work within California, then you will have no income tax obligation to California and therefore no need to make California estimated tax payments.
But if you're an independent contractor (with no taxes being withheld from your pay), and you have clients located in CA, then yes, you should make quarterly estimated tax payments to CA. An independent contractor receives a 1099-NEC at year's end, not a W-2. CA taxes independent contractor income earned from services provided to CA-based clients.