Updating company field

Hi - not sure I 100% understand the question - are you asking for how a form fillout impacts the lead record in Marketo, or the lead/contact in Salesforce? (Since you are using the word Account I suspect the latter, but don't want to assume...)

I don't think it updates every lead, but I'm not sure and I'll be interested to know. As another question for the Marketo Community, what is the best way to update information by company (besides running a Smart Campaign...)?

By default Marketo does not update account object fields.  This article can provide more nuance:
https://community.marketo.com/MarketoArticle?id=kA050000000KzbQCAS

It depends. If the Lead is an SFDC Lead, it only applies to that record. However, if the Lead is an SFDC Contact and Company Name is changed via a form submit, this will show for all Leads that are also linked to that same Account. However, this will NOT sync to SFDC (unless you have asked Marketo Support to enable writing to the SFDC Account Object). Also, as soon as there is an update to the Account record in SFDC, the Account Name from SFDC will be copied to Marketo again, overwriting the company name that was submitted in the form. 

If you want to avoid this behavior, create a custom Marketo-only field "Company for on Forms" and have a Smart Campaign copy this over to the Company field for Leads (but not for Contacts).