Not all ISPs provide routers as part of their service; it's down to individual contracts.
My own ISP (Claranet) doesn't provide routers; the last time I asked about them (a few years ago) they said that they could sell me a pre-configured one for £86 but that, if I was happy set one up myself (which is very easy with most modern routers, as they do almost everything for you anyway), I could save money by buying elsewhere. (I then bought an excellent router online, in Maplin's closing down sale, for about twenty quid and set it up quite easily).
Routers are basically the same (irrespective of the ISP you use) but
(a) some are dual frequency (which
might be relevant in some situations:
https://computing.which.co.uk/hc/en-gb/articles/360001141160-Understanding-dual-band-wi-fi-routers ) ; and
(b) some are better at getting a signal to remote areas of a house than others are. (That could be important if you live in a big house with solid walls but totally irrelevant if you live in a small flat or a tiny house like mine).