It measures websites; Skomi also measures apps. Fathom publishes no mobile or desktop SDK. Its documentation and feature list are web only. Skomi ships SDKs for eight platforms against a frozen wire contract, so a site and the app beside it report into one account rather than needing a second product and a second bill.
Behaviour comes in the same subscription. Heatmaps, session recordings and a feedback widget are Skomi modules, metered from the same credit allowance the analytics uses. With Fathom each of those is somebody else's product.
There is a free plan, not a trial. Fathom gives you seven days and then asks for a card. Skomi's Hobby plan is free and stays free: one site, one app, ten thousand credits a month, with every feature switched on.