Which epoxy should I use to embed objects, and do I seal them first?

It depends on how deep each layer needs to be. UltraClear Bar & Table Top Epoxy pours 1/8" per layer, with additional layers every 4 to 6 hours, so it is the product for items that sit on or just under the surface, such as buttons, shark's teeth or a paw-print casting. UltraClear Deep Pour Epoxy is for layers 1/4" and deeper; it will not cure properly thinner than that. Pour it in 1/4" to 1" layers every 12 to 24 hours, up to 6" total, for stones, toy cars, corks, game pieces and other tall objects.

Porous items need a seal coat first. Wood substrates, corks, old flour-and-water castings and anything else porous should be brushed with Bar & Table Top Epoxy before epoxy goes over them; otherwise they release air bubbles into the pour. Plants and flowers embed well as long as they are completely dry and free of dust or debris. The heat from curing epoxy is typically not significant enough to harm embedded items. Loose sand has to be held in place: brush the seal coat onto the substrate, spread the sand, then spray adhesive over it so it does not float when you pour.

If the piece will be a bar, table or countertop, finish with a Bar & Table Top flood coat once the Deep Pour has cured, because Deep Pour is not a durable wear surface. Test small first if you are new to epoxy.

Using both products together: https://ultraclear.helpscoutdocs.com/article/859-table-top-epoxy-deep-pour-epoxy-river-table