I think you meant RJ11. Ethernet uses RJ45.

But that's just the jack. Either can use CAT5/6/7 cabling and most RJ11 in more recent homes is still CAT5 or better. But that doesn't mean the builders didn't staple it to the studs making it worthless for anything more than 10BASE-T (if even that).

Might be worth having someone run cat6e in a similar fashion though. I had it done in my previous house and it was glorious.

Mine had some wiring for it when i moved in in 2014, but we never used it and removed them as we came across them since we knew its not needed ever again at this point. Same with coax for cable tv which we did use for a few years but switched to streaming and from what i see even cable boxes are just wifi and connect back to the demarc point were they installed the main box. Only one we kept was the coax coming into the house for internet which is or can still be used for now.

With that said i absolutely want to run CAT6 cable through the house were the basement has 1 spot with a switch that has 1 feed to the main floor back to the demarc and same for the 1st floor. I hate wifi and sure its great for phones and stuff, but if i can have my tv streaming boxes, pcs, and gaming consoles on wired then i absolutely will do that instead. As someone who works in IT, the number one complaint i see is employees working from home and cant figure out why voip calls or meetings sound awful until i point out their terrible wifi situation from either not being close enough to it or still using that 10 year old service provider all in one router.