The Wikipedia section on the Election method for the 2021 Scottish Parliment describes the set up as;
The Scottish Parliament uses an Additional Members System, designed to produce approximate proportional representation for each region. There are 8 regions, each sub-divided into smaller constituencies. There are a total of 73 constituencies. Each constituency elects one MSP by the plurality (first past the post) system of election. Each region elects 7 additional MSPs using an additional member system. A modified D'Hondt method, using the constituency results, is used to elect these additional MSPs.
This suggest a single vote is cast and the FPTP vote counts are used to measure proportionality for the list vote. Alternatively, several places including this BBC article state Alba will be running a list-only strategy, and the SNP will continue to run a pragmatic both SNP campaign.
[Alba Party Leader Alex] Salmond believes this means most SNP list votes are "wasted", and could instead be given to his new list-only party - which would have zero constituency MSPs, and thus a better chance of winning regional seats.
They are unlikely to win every single constituency, and thus must try to balance out any local defeats with list seats - from a pragmatic standpoint, their campaign has to be "both votes SNP".
This seem contradictory, either the list seats come from a second vote (as in the German set up) or they are based on the constituency vote. Which is it?