Learn Swedish: Vi uppskattar tryggheten här. - We appreciate the security here.

It is built from the adjective trygg (safe/secure) and the suffix -het, which works like "-ness" or "-ity" in English to create abstract nouns. Finally, the -en at the end makes it definite ("the security").

Is trygghet an en-word or an ett-word?

It is an en-word! One of the great things about the -het suffix is that all nouns ending in -het are en-words. That's why "the security" takes the -en suffix to become tryggheten.

Could I use säkerheten instead of tryggheten here?

You could, but it changes the vibe slightly. Trygghet usually refers to a feeling of emotional safety, comfort, or being free from worry (like feeling safe in your community). Säkerhet refers more to physical or objective security measures (like alarms, locks, or seatbelts). Here, they appreciate the safe, comfortable atmosphere.

What is the infinitive form of uppskattar?

The infinitive is att uppskatta (to appreciate). It is a very regular Group 1 verb, which means it ends in -a in the infinitive and simply adds an -r to form the present tense (uppskattar).