The 2020 remake rebuilt the first two games in Unreal Engine 4. Unreal games are relatively approachable to mod: assets ship in .pak archives that the community can unpack, edit and load as overrides.
THPSPro
The best-known mod is THPSPro. It brings in levels from across the series, rebuilt for the remake: THPS3 and THPS4, THUG 1 and 2, the Project 8 world with Downhill Rio, and some American Wasteland levels. It also adds custom restarts, a free-roam camera, unlocked hidden cheats, classic skins, and board and griptape graphics. It even has a limited online mode: free skate over LAN or a VPN, on the Epic version only (Nexus).
Its author, bAstimc, also brought THPSPro to THPS 3 + 4. The full story is in the THPSPro history.