You have three basic types, 1) just sit in the hole 2) can be retained by a screw from underneath and 3) can be secured from above
Bemchdogs do these that just drop in the hole
Bench Dogs Large x 2
They do these quad dogs which are multipart and use o rings to lock into a hole by just tightening
https://benchdogs.co.uk/products/quad-dogs™-40mm-pair
The choice is yours and your needs will probably evolve as you use them, I have a row of dogs fixed from below along the back of one of my worktops that just stay there and come in handy for many jobs.
It can be better to make your own tops because they will not last forever, I think of them as being disposable and something that will get cut up so I use the Parf system for making them. Search these forums and you will find a lot of info on dogs and benches, there is some really good experience that has been shared. For me I had initial reservations about having lots of holes in a worktop and just how useful could they really be, my solution was to use boards with the holes that sit on my main workbench, if I need them I lay them down else take them away and now a solid bench where nothing can drop through.
What really changed things for me was the Microjig system, it gave a bench with dogs even more usability because now the dogs align and ensure workpieces are in the correct position whilst the microjig slots allow clamping and this has been such a help. Also when using the Parf system to make the top you can get a row of holes in and then use these to run a parallel microjig slot which allows you to clamp the Parf jig whilst drilling the next holes and I found this gave tighter holes.
Look at this bench for an idea but make the holes 20mm for dogs, now you can align workpieces for joining in two planes.