Next to the battery you'll find the m.2 SSD - it's about 2 cm by 8 cm and yes, it is a card.

These systems do appear to support a 2.5" drive as well -- the qualifier:  if you have the larger of the two batteries (4 cell vs 3), it may block the drive bay and mean you can have only the M.2 SSD.  Even if the space is open, it requires a carrier and cable assembly to connect a 2.5" drive.

From a performance standpoint, you'll want to use a 1T M.2 2280 NVMe SSD to replace the original one.  2.5" SATA SSDs are appreciably slower.

Service manual:

https://www.dell.com/support/home/en-us/product-support/product/latitude-15-3500-laptop/docs

Hi ejn63

thanks for helping, sorry about the url is amazon link...  

something like this https://www.amazon.co.uk/addlink-Gen3x4-3000MB-Internal-Solid/dp/B07PM347GV/ref=sr_1_1_sspa?adgrpid=63656212792&dchild=1&gclid=CjwKCAiA6aSABhApEiwA6Cbm_2NbyiQbxUUK4_pkeEzV22pqCvFfD0NDutqHwTlRBJnnf_1JtTQ9MxoCR_YQAvD_BwE&hvadid=317544485691&hvdev=c&hvlocphy=9046993&hvnetw=g&hvqmt=b&hvrand=10372902182083365737&hvtargid=kwd-310274098707&hydadcr=19077_1719658&keywords=m2+2280+ssd+1tb&qid=1611259803&sr=8-1-spons&tag=googhydr-21&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUExMTFKWlVNRktCTlI1JmVuY3J5cHRlZElkPUEwNzU0ODc1M1I2TEhPQllZV1k0MyZlbmNyeXB0ZWRBZElkPUEwNDI1NzQ4MlUwQThaRUgwV1MyRSZ3aWRnZXROYW1lPXNwX2F0ZiZhY3Rpb249Y2xpY2tSZWRpcmVjdCZkb05vdExvZ0NsaWNrPXRydWU=

Hi ejn63

Thanks for helping, currently thinking about the Samsung 860 EVO 1TB M.2 SSD, do you think this wil be fine?

Thanks

John 

https://www.amazon.co.uk/Samsung-Sata-Solid-State-Drive/dp/B078WQT6SB/ref=sr_1_2?adgrpid=109861860363&dchild=1&gclid=CjwKCAiA9bmABhBbEiwASb35V9X-GTzc3JbsqbEEw3H7G1du_HS2S6iX26NRUwCf-6D_NdCvYBuQHhoCUH4QAvD_BwE&hvadid=469301638967&hvdev=c&hvlocphy=9046993&hvnetw=g&hvqmt=e&hvrand=6410742298243673187&hvtargid=kwd-474424717865&hydadcr=23913_1814600&keywords=samsung+1tb+evo+860&qid=1611590351&sr=8-2&tag=googhydr-21

According to the manual, yes - you can use an M.2 SATA drive.  However, for better performance, NVMe would be a better idea than SATA (and SATA m.2 support is rapidly vanishing from new systems, so if future-proofing is a goal, the added cost for NVMe is likely worth the expenditure now).